since no one is even venturing a guess, here's the answer...

at the mi'ihen fiend colony (secret dungeon) the one with the insects. you have to blast a fixed wall and you'll get this accessory. there are other tomes in this area and you can get them from chests, the bushido tome is the only one you get directly from blasting a wall. (also try blasting walls and you'll get gil, potion, the occasional fiend battle, etc.)

anybody want to take over with the next question?