Taken directly from the
Warlock Information Compilation:
Itemization for a Warlock... by cheezgrater
If I don't list a sourcebook, then it is straight out of the 3.5DMG
Chasuble of Fell Power - boosts damage from your eldritch blast. Buy this. (Complete Arcane)
Warlock's Sceptre - Gives you +2 to hit with ranged touch attacks, and by spending charges you can boost your eldritch blast damage. (Complete Arcane)
Cloak of Charisma - Useful instant boost to your save DCs if you use a lot of save-based invocations
Gloves of Dexterity - Useful for blaster locks and meleelocks that rely on weapon finesse.
Boots of Elvenkind - Warlocks get invisibility invocations. Move silently bonuses can't hurt for those times when sneakiness is the order of the day.
Mithral Full-Plate - with the Battlecaster and Medium Armor Proficiency feats, you can get a really nice AC for your warlock, with no spell failure chance. Enchant this for even better armor.
Belt of Magnificence - Expensive as all get-out, but boosts all 6 stats while taking up only one slot. IIRC, it's 200,000gp for the +6 version, but +6 to every stat is really nice. (Miniatures handbook)
Cloak of Displacement, Minor - all attacks against you have a 20% miss chance. Pretty nice, especially for those warlocks that like to get into melee. This can serve as a replacement for the Entropic Warding invocation (and the item is better since it also works on melee attacks), except that it doesn't make you untrackable
Pale Green Ioun Stone - +1 on attack rolls, saves, skill checks, and ability checks that doesn't require an item slot? Yes, Please!
Orange Ioun Stone - +1 caster level, slotless. Won't give you extra invocations or boost Eldritch Blast damage, but can be useful if you see many enemies with spell resistance.
Manuals and Tomes - +5 inherent bonuses to Dex and Charisma are really nice for a blaster. Con is also nice for a melee lock What more needs to be said? You may also want to consider other books if you have money to burn. Beefing up stats is rarely a bad thing.
Horizon Goggles by FromTheShadows - I'm surprised nobody really noticed the Horizon Goggles in Complete Mage. For a mere 8,000 gp, you get extra range on your blast or invocations as long as they happen to be in a form that uses a ranged attack roll.
Codex Advocare by Tweedledope - Ya'll don't forget the Codex Advocare from the Expedition to Castle Ravenloft. It might seem minor but, for 20k it gives you an extra Least invocation known. Sure beats wastin' a feat.
You might also get some use out of
X stat to Y bonus.