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?