There are many useful items out there for scouts, and many of them are quite cheap. Don't overspend here... you still have to be useful to the party once the scouting is done. Mundane items are first, magic second. Mundanes item have the advantage that if you can craft them (with Fabricate or just a lot of time) you can get them for 1/3 cost.
Skill Boosting Items in general: These have a set formula for a generic bonus to a skill. You'll probably want some for Hide, Move Silently, and Search. I won't bother listing them all here, but they're a good idea to get in general... and remember, different bonus types stack.
Amulet of Fortune Prevailing (MiC): For 5kgp, this item lets you reroll one save per day. Simple, and it works.
Amulet of Second Chances (MiC): Though expensive at 40kgp, this item completely undoes one round. That's enough to avoid any ambush or trap. Not bad.
Aspect Mirror (CSc): For 4kgp each these allow for both visual and auditory communication. Just leave one with your party and keep one with you, and you can show your party everything you're seeing (or whisper to them, or use hand signs to communicate). Your party can even scry on your location with their mirror (though if you're far enough away that that's necessary, you may be in trouble).
Belt of Battle (MiC): For 12kgp, this item grants +2 to initiative and can give a few extra actions... enough to seriously change the opening rounds of any one fight.
Blindfold of True Darkness (MiC): 9kgp buys you an item that grants 30' Blindsight, but you can't use normal vision. It's handy to keep around, but due to limited range it shouldn't be worn all the time. Those folks who already have Mindsight might find it useful enough, though, especially if the DM has stated that Mindsight works on everything.
Boots of Bigstepping (MiC): 6kgp for the ability to teleport 60' twice per day isn't bad, though the Shadow Hands might be a better choice.
Boots of Sidestepping (Dun): For 3kgp, these boots let you move 5' any time you would need to make a reflex save as an immediate action. If this pulls you outside the area of effect, you simply dodge and don't even need to make a save. It's basically immunity to many traps, in addition to a number of other effects (remember that if you dodge behind cover you'll gain cover bonuses). Note the MiC version of these boots is twice the cost and not nearly as good.
Boots of Striding and Springing (DMG): For 5.5kgp, these give +10' to your run speed and a +5 to jump. Having a higher speed than the average monster can be quite useful if you find yourself in a bad spot, so these are likely worth it... especially if you're already a Swordsage with Tiger Claw maneuvers.
Charming Eye (Fiend Folio): At 120kgp this thing is horribly overpriced, but at higher levels it's free... if you can get someone to summon a Sibriex (Fiendish Codex 1) they'll make up to three Fiendish grafts including this one free of charge. You have to be evil to use it (if you're not it'll corrupt you towards evil) but it lets you see just fine in all forms of darkness, including magical darkness. There's no range limit for this ability, which makes it one of VERY few ways of seeing more than about 200' in darkness, and since it works in magical darkness there's a few other interesting tricks you can use this for. It also comes with a DC 14 charming gaze attack... unlikely to matter except if a critter rolls a 1 on the save, but hilarious when it works (remember any creature attacking you will have to make this save).
Chronocharm of the Laughing Rogue (MiC): It's only 500gp for a once a day reroll on a failed disable device roll, even after seeing the trap go off. Not bad at all, and plenty cheap.
Cloak of the Bat (DMG): It's pricey at 26kgp, but this cloak does give a +5 bonus to Hide and the ability to turn into a (Tiny sized) bat and fly around. That size thing is a big (heh) bonus when you're trying to be sneaky, and unless you have some innate ability to do it having the ability to fly is something you needed anyway.
Collar of Umbral Metamorphosis (Tome of Magic): This amazing piece of equipment is valuable to any scout who doesn't already have the Dark template. It simply grants the template for a short time per day for 10kgp or continuously for 22kgp. Since the template includes movement speed, hide in plain sight, improved vision, and boosted stealth skills, it's an automatic choice.
Contact Medallion (MiC): 3kgp lets you telepathically contact anyone you know within one mile for a minute 3/day. Should be enough to contact your party if you have no other means and need to. Technically if you have Mindsight this might let you automatically see the location of that one person when you use the ability, if that matters.
Collar of Perpetual Attendance (Online Supplement): Yes, this is an official item (google it!). For 2kgp you get to cast Unseen Servant at will. Consider springing traps, pulling levers, opening doors you've already unlocked, grabbing possibly dangerous items, and generally doing anything dangerous of that sort.
Deathward Armor Enchantment (MiC): For a +1 Enchantment, this one is a great ablative defense. Once per day (as an immediate action) you can ignore any death effect or negative energy effect. It's cheap enough to be well worth it.
Diamond Eye Circlet (MiC): 2kgp gets you Detect Undead at will. If you were using Lifesense, now everything but constructs is automatically detected for you.
Dimension Stride Boots (MiC): These 2kgp boots are another teleport escape option... they'll send you 20 feet 5 times a day (or farther fewer times per day). It's just one more great way to get out of grapples and such.
Eager Weapon Enchantment (MIC): This +1 Enchantment lets you quickdraw the weapon, but more importantly grants a +2 to initiative. That's always a big deal... but if you don't care about the quickdraw aspect, Warning is the higher priority.
Easy Travel Armor Enchantment (MiC): For 1500gp this lets you hold a medium load as though it were light. Many scout types have trouble carrying everything, and this helps a great deal... and it's quite cheap.
Energy Immunity Armor Enchantment (MiC): This +2 bonus gives you a once a day immunity for one minute to any energy type of your choice as an immediate action. It's a great way to prevent one attack from landing.
Enveloping Pit (MiC): At only 3,600gp, this is one of the cheapest significant portable spaces around, and it's huge (it's 50 feet deep and 10 feet square). The relic power is cute but not necessary at all. What's great about this is that you can hide the entire party in the pit (make sure they won't have air problems if it will be a while) and sneak them past all kinds of obstacles safely. Combine it with a Glove of Storing or Gloves of the Master Tactician so you can instantly pop it open when you need and now your party is where ever you wanted them to be. You must be within one step of Lawful Evil to use it though. Otherwise, you'll need to stick with a Portable Hole.
Fast Leg (Fiend Folio): 4kgp for an untyped 10' boost to movement speed is quite handy, and it gives a +5 to climb and jump as well. It's a fiendish graft though... you have to be evil to use it, but you can get it for free from a Sibriex (FC1).
Fearsome Eye (Fiend Folio): Like the Charming Eye above, this graft is so expensive that no one in their right mind would ever buy it... but using Planar Binding to get a Sibriex (FC1) will get it permanently for free. It grants permanent See Invisibility along with a likely completely useless (at the level you'd be getting it) fear effect. But hey, if you happen to be at such a high level and happen to fight something that's not immune to fear, having it get paralyzed when it rolls a 1 on the save while attacking you would be really funny. Either way, if you didn't have Permanent Arcane Sight or similar already and are evil, you might as well score this.
Feathered Wings (Fiend Folio): For 10kgp, these wings give a decent flight ability (Average, twice your ground speed). You also get a free buffet ability. It's a nice cheap way to get permanent flight, but you do have to be evil. Like the other fiendish grafts, you can get it for free from a Sibriex.
Gloves of Object Reading (MiC): These 3kgp items are vastly underestimated and a must have. If you're sneaking into a dungeon and you loot a chest, whatever is in that chest likely belongs to someone important in that dungeon. You can now ask questions like "what traps did he have to avoid when walking around the dungeon" and "what password did he use to get past the green door?" Used right these can function as a map to the entire dungeon.
Golembane Scarab (DMG): 2500gp lets you detect golems as a standard action, and you ignore Golem DR. If you have Lifesight and the Diamond Eye Circlet, there's very little that you don't instantly know about.
Gwaeron's Boots (MiC): At 6kgp, these boots give permanent pass without trace, and make you undetectable to scent. Without Pass Without Trace, even a seriously stealthy character could still be tracked.
Hand of the Mage (DMG): For just 900gp this item lets you use Mage Hand at will, letting you trigger traps, make sounds far away from yourself, and generally get creative. Considering how cheap it is, you really should get this.
Hat of Anonymity (MiC): Pricey at 12,500gp, this hat gives an untyped +5 bonus to hide (nice) and permanent nondetection (really nice!).
Headband of Conscious Effort (MiC): 2kgp lets you make a concentration check instead of a fort save 1/day. Nice, but consider also the Novice Ring of the Diamond Mind, which could teach you the Moment of Perfect Mind Maneuver.
Helm of Telepathy (DMG): Detect Thoughts at will is handy, and so is the telepathy this gives. And hey, it's a way to throw Mindsight on a non magic character! It's 27kgp, though.
Intelligent Items (DMG): If you can get one, these can be surprisingly useful. +12kgp gets you one with 120ft Blindsense and Darkvision, and it can observe for you while telepathically informing you of anything important. Other useful available abilities include Detect Magic at will (3600gp) and Detect Thoughts at will (pricey at 44kgp). If you're close to Neutral Evil, consider the Dagger of Denial from MiC for a handy example.
Iron Heart Vest (ToB): The Novice version of this item costs just 3kgp. If you can double it up (7500gp if your DM allows it) for two maneuvers, you could get Iron Heart Surge once per encounter... absolutely worthwhile. No sense letting one debilitating shot take you out of the fight.
Linked Armor Enchantment (MiC): Though expensive at 6kgp, and both you and a party member must wear such items, this lets you communicate with your party within 10 miles easily and silently. If you lack telepathy, this is one way to get the job done.
Lockpicking Ring (MiC): This 3500gp item gives a +5 to Open Locks... but more importantly, it casts Knock once per day without need for UMD. It's one of few ways certain scout classes can deal with Arcane Lock (Ninjas, for example).
Mask of the Tiger (MiC): If you don't have Low Light Vision already, you want it. This 2kgp item will get it, and you get the Track feat free (or +5 Survival if you had it already).
Medallion of Thoughts (DMG): It's only 12kgp for a Helm of Telepathy without the actual Telepathy part. Detect Thoughts is quite nice if you wanted to learn more about what's waiting for you in the dungeon, and if you already had telepathy some other way, this is much cheaper.
Ring of Anticipation (DoU): +2 Competance to Spot and Listen for 3kgp is a rip off, until you notice the other effect... re-roll any initiative check. That's a huge effective boost to initiative and shockingly cheap.
Ring of Chameleon Power (DMG): At 12kgp this one's a little pricey, but +10 hide and unlimited Disguise Self is extremely handy. Combined with Bluff this can let you seem to be on the side of your enemies, which can be quite useful...
Ring of Communication (MH): You'll need at least two of these 2kgp items, but they let the wearers talk to each other with a range of up to a mile. This is the only all day communication item like this, and they're quite cheap.
Ring of Darkhidden (MIC): For just 2kgp this ring grants total invisibility to Darkvision. Take a look at a lot of creatures designed to function at night or underground... many of them rely on Darkvision alone as their means of finding anything. This ring makes them totally helpless against you, and is so good the rest of your party should probably have one too. Plus, this way you don't have to worry about Darkvision creatures being able to spot you due to lack of shadows to hide in. It's too cheap and too good not to take as soon as it becomes relevant unless you're always running around in daylight.
Ring of the Diamond Mind (ToB): The novice version is just 3kgp and lets you have Moment of Perfect Mind, which when combined with a decent concentration score means one automatically passed will save per encounter. If your DM lets you double up the ring (or if you just have one on each hand) you could get Mind over Body too to pass a fort save. Even added into another ring for 4500gp this is absolutely worthwhile.
Ring of Mind Shielding (MiC): For 8kgp this item hides your mind from many detection abilities. Ask your DM if it protects you from Mindsight (if that's in play). If so, this is absolutely amazing.
Ring of Mystic Defiance (MiC): For 7500gp this item lets you take an immediate action to reduce the damage from any spell or spell like ability by 10, and lets you add up to +3 to fort saves against spells and spell like abilities if you have an item that boosts Int or Charisma that much.
Ring of Nullification (ToM): A great example of ablative defense, the Ring of Nullifcation costs just 3,200gp and nullifies up to 20 damage from an attack type of your choice. "When I'm hit by a trap" is an obvious one to go with, and should insure you live through one otherwise nasty attack.
Ring of Spell Battle (MiC): For just 12kgp this item automatically informs you of all spellcasting that occurs within 60 feet of you even if you can't see it, and lets you use Spellcraft to figure out what the spell is. It's great for knowing if you're about to get attacked and what the capabilities of enemies are. Plus, you can then counterspell one spell per day or redirect the target of any one spell... REALLY nasty.
Ritual of Shadow Walking (LoD): Technically not a magic item, but close enough: this ritual costs 2k Xp and 500gp, and gives you the ability to travel through shadows at will up to 100ft. You can also hide in shadows indefinitely. It costs 2 temporary con damage and 2d4 damage to teleport, or 4 con and 3d4 damage per round to hide in the shadows. Still, it can get you out of a jam, and if you happen to be binding Naberius (via a dip into Binder) you can heal the con damage immediately... or if you're immune to con damage anyway you can have all kinds of fun.
Sandals of the Light Step (MiC): This is pretty expensive at 9kgp. But you get +10' to movement speed, Pass Without Trace, and a complete lack of difficult terrain penalties. Quite handy at times.
Scrolls of Uncertain Provenance (MiC): These 8kgp scrolls require that you be within one step of Lawful Neutral, but at 8k they provide a +5 to all Knowledge checks just for possessing them. They have relic powers too but those are less relevant. These can be quite handy for making sure you actually know your enemy, and are an obvious choice if you took Knowledge Devotion.
Shadow Hands (ToB): 3kgp gets you one maneuver from the Shadow Hand school. Shadow Jaunt is a great idea, though Cloak of Deception works too. Even once per encounter it's a great escape, and it's very cheap.
Shadow Cloak (DoU): This 5500gp cloak gives a +1 deflection bonus to AC, and more importantly three times per day you can either teleport 10' or gain concealment as an immediate action in response to an attack. Any sort of attack. It's much like the Conjurer Wizard trick, and can let you completely spoil one attack against you.
Shirt of Wraith Stalking (MiC): This 6kgp gives you a permanent hide from undead effect... and intelligent undead can't even make a save. You're just impossible to see for them.
Silent Portal Disk (MoF): For just 350gp you get a disk that can be attached to any door to make it totally silent. Not bad for sneaking around.
Slicksilk Belt (MiC): 2kgp lets you use Escape Artist checks as a Move Action and gives a +10 boost once per day. Usually that's enough to help get out of grapples safely, and if it fails you've still got a standard action.
Slippers of the Setting Sun (ToB): 3kgp for Counter Charge once per encounter? Yes please. It's cheap enough that adding it to any existing boots for 4.5kgp is absolutely worth it. Consider adding it to the Steadfast Boots... now if something charges you while you've got a reach weapon, you can hit it with a readied attack, then hit it with an attack of opportunity, then redirect the creature away so it doesn't hit you at all. Total cost for such boots? Just 5,100gp.
Steadfast Boots (MiC): These are quite cheap at 1,400gp, and as long as you're holding a two handed weapon you automatically count as having it readied against any charge against you. Combined with Improved Trip or some other ability to spoil charges, and these boots will completely ruin one attack against you if your cover is blown. They also help protect against tripping, bull rushing, and overrunning.
Stone Dragon Belt (ToB): The novice version of this costs 3kgp, and gives you the Mountain Hammer maneuver once per encounter. Ignoring hardness means smashing through anything. This can open up all kinds of unexpected routes.
Third Eye Clarity (MiC): For 3kgp this item lets you negate any of the following conditions: Stunned, Confused, Dazed, or Fascinated. You can chose to use it after finding out you've failed the save for the effect. Considering how much these conditions screw you, this is quite nice.
Third Eye Conceal (MiC): At 120kgp, this is an extremely expensive item only appropriate for very high levels. However, it protects against Mindsight (as well as all other forms of mental based detection), so in some games it may be the only way to do so. Warlocks with CWI can craft it a bit earlier, as can Artificers.
Third Eye Freedom (MiC): Cheap at just 2,600gp, this item gives you a round of Freedom of Movement once per day as an immediate action. That's one guaranteed escape from a grapple or other binding effect.
Third Eye Sense (MiC): 24kgp is a steep price, but this item lets you at will see from any location either familiar to you or obvious to you (such as behind a door that you can currently see, around a corner, etc). The uses for this are obvious... remember the usual penalties for spotting go away if you can see from right next to the target! This is also very handy for archers, who otherwise have trouble seeing targets from far away.
Torc of Displacement (MiC): This immediate action item runs at just 2kgp and lets you gain a 20-40% miss chance for one round when you need it, depending on the number of charges used (it has 3 per day).
Trackless Boots (DoU)): These boots are just 1kgp and while wearing them scent can't detect you, plus people tracking you are at a -5 to do so. This is one nice way to deal with patrol dogs and such if your DM uses them against you, and is especially nice if Darkstalker isn't available to you.
Wand of Knock (DMG): For 4,500gp this item means Arcane Lock doesn't outright screw you. It's critical for Rogues, and useful for others.
Warning (MiC): This +1 enchantment gives +5 initiative. Simple, cheap, effective. Don't leave home without it.