[PF] Flurry of Blows with a non-monk weapon

[Annabeth]

Expanding the list of weapons to use with Flurry of Blows:
1. Ascetic Form. Use the versatile design mod to add any melee weapon to the monk weapon group. Often a Weapon Adept or Modified Weapon Proficiency feat is also required to wield a modified weapon.
2. Crusader's Flurry feat, allows you to treat your deity's favored weapon (melee only) as a monk weapon. For prerequisites, a good option is dipping into the Crusader archetype for the cleric.
3. Sohei Monk can use FoB with any of the following weapon groups: bows, crossbows, monk weapons, polearms, spears, or thrown weapons. Versatile design will further expand the list of possible weapons.
4. Student of Perfection will allow you to select the Ki power Genie's Blade, and treat the falchion, kukri, or scimitar as the monk's weapon.
5. Kyton Style allows the spiked chain to be considered as a monk weapon.
6. Feral Combat Training for FoB with natural weapons. You can also take Martial Versatility feat to use FCT with all weapons in the natural weapon group. Another way is to choose FCT with unarmed strike, since the monk's weapon is considered a natural weapon, and only then try on the MV feat. This will give access to the FoB with weapons from the following groups: Close, Monk, Natural, Tribal. Shenanigans with Versatile Design are also possible. By choosing the Martial Artist monk archetype you will be able to do this without multiclassing.
7. Catfolk favored class bonus allows claw blades to be treated as monk weapons.
8. Various monk archetypes.
9. Blade of the Sword-Saint katana, stylish, expensive.
10. Wyvern Fury Style for FoB with dagger, punching dagger, scorpion whip, spiked gauntlet, or whip.
11. Spear Dancing Spiral allows you to use any feat or ability with the selected weapon as if it were a quarterstaff. Not very useful since it has TWF in prerequisites, but it exists as an option for FoB with weapons from polearm or spear fighter weapon groups.
12. Weapon Shift feat will give all of your natural attacks when using Wild Shape the properties of the selected weapon - this can be used to give them the "monk" special quality.

What other options exist?

[Power]

Adherent of Ancient Osirion religion trait (probably requires you to worship Bastet) lets you treat tekko-kagi as claw blades, which combined with the Catfolk favored class bonus for Monks lets you count it as a monk weapon and even use it to make style strikes as an Unchained Monk.

Ascetic Form + versatile design greataxe + Fiendish Obedience (Haagenti) using either Sentinel or Damned Soldier lets you flurry with all weapons. Sentinel has 7 BAB prereq when used with Fiendish Obedience though, according to the sourcebook. You don't actually need to be within 1 step of Haagenti to take Fiendish Obedience or Damned Soldier, and also you can also get a Flurry of Blows as a Sacred Fist Warpriest, which does not have the Lawful alignment requirement. In addition, Haagenti likes to pose as a more helpful and reasonable inventor, so it's not hard to come up with a reason to worship him outside of 1 step of CE. However, you will only unlock this stunt at level 16 if you use Damned Soldier, but getting Alchemical Allocation 2/day as a SLA at 12 is also good and the capstone is a buffed Shapechange which is a pretty decent prize for a martial at 20, especially if you are a Scaled Fist Monk and thus have a high charisma, boosting the DC of shapechange and, by extension, all special abilities obtained through polymorphing.

[Nanashi]

Actually

A sohei may use flurry of blows and ki strike with any weapon in which he has weapon training.

You can actually flurry anything that has a weapon group with this, you just need to get weapon training for other groups from a different class. Fighter 5/Sohei 6+ is doable (not good, just doable). Arguably the Ninja Trick "Weapon Training" qualifies.

[Power]

Weapon Master Fighter 3 would be faster for that, but it would only give you a single weapon. A Variant Multi-Class Fighter gets Weapon Training 1 at level 11 though.

Honestly, dipping 1 level of Cleric and using Crusader's Flurry and Guided Hand is probably drastically superior as a dip for a Monk, although Guided Hand is a waste if you can just use the Guided weapon property. If you VMC Cleric (or Life Oracle, though I do not recommend it), which is a fairly thematic multiclass for a Monk, you won't need to dip 1 level, and you can get some rather useful domain powers at full character level. If you take Exotic Heritage (Empyreal), you can also obtain a Channel Energy at level 11, but the charisma requirement means you should be playing a Scaled Fist and it will cost you 3 feats (however, you can probably use the 2nd feat to obtain a Bloodline Familiar that makes a decent healer instead of the crappy Heavenly Fire ability).

Although I doubt the Weapon Training rogue talent or ninja trick would qualify (since it clearly isn't what we would conventionally regard a weapon training feature, as the only thing it does is give a Weapon Focus feat of your choosing), if it does, you can dip either Pathfinder Field Agent or Rose Warden for 1 level to obtain it instantly.

[Annabeth]

I doubt weapon training rogue talent qualifies either, but if that's the case, using the Weapon Training trait might work as well.

[Power]

Aye.

Another quick stunt: Fiendish Obedience (Haagenti) (probably with Damned Soldier) can also be used with Crusader's Flurry to allow you to treat all weapons as monk weapons. Still, this isn't very practical, since it only pays off at level 16.

One simple way to Flurry of Blows with a weapon of your choosing is to just use Crusader's Flurry as a Sacred Fist Warpriest, which does not require multiclassing or a lawful alignment. You just have to remain within 1 step of your deity. Only having 3/4 BAB while flurrying is a pity, but getting to cast Divine Favor as a swift action + increasing it with Fate's Favored faith trait easily makes up for it. However, Crusader's Flurry might necessitate a 3 feat chain (weapon proficiency, weapon focus, crusader's flurry) depending on your weapon and racial proficiencies.

[Power]

Wyvern Fury Style only allows you to make 1 of the attacks with a whip or scorpion whip. Also, the dueling dagger, as a special property, applies any effect that applies to a regular dagger to itself, so it can be used with Wyvern Fury Style also.

As for the Blade of the Sword-Saint (75,350 gp), if you upgrade it with the Greater Transformative property (15k gp), you can transform it into any weapon (even a ranged weapon, but not ammunition) while still retaining its properties, including the special magic ability that lets you treat it as a monk weapon. Honestly though, you're going to want someone to craft it for you with a Craft Magic Arms and Armor feat (or perhaps do it yourself with Master Craftsman and Profession (Blacksmith)) in order to obtain and afford the weapon.

And depending on how you read Martial Versatility, you can use it with Kyton Style to be able to treat all weapons in the Flail weapon group (which includes whips) as monk weapons. The two paths to gaining a Fighter level for the purposes of the feat include playing a Sacred Fist Warpriest (which has a Fighter level for purposes of its bonus feats) and dipping one level of Fighter before advancing as a Student of Perfection, which stacks its class levels with any classes that have the Bonus Feat class feature for the purposes of which feats are available while also continuing to advance flurry of blows, unarmed strike, fast movement, etc. It's worth noting that many races (Aasimar, Tiefling, Oread, Undine, etc.) have an alternate race trait to count as humans as well. And of course, once you do this, you have yet another path to using the versatile design weapon modification to make a weapon count as a monk weapon.