I want to craft the mitical spear Gae Bolg (just to have it), and i need one of this lvl 49 odachi, but i don't know where to
find it. Does anyone have a clue of where it is dropped?

I think it must be around kyoto or sekigahara, but i will thank if someone could tell me the dungeon where it is dropped.

Best thing I can say is just farm an area that is around the level of the said weapon you are trying to find and just pray to RNG when you go Identify it. Far as I can tell weapons don't have a certain place where they drop.

I found mine at den of needles