pls help!! in which dungeon????/

Kishinden, at a very low rate.

supremetentacle i think i got the wrong weapon its a lvl 71++ weapon =/

merlins den, goukiris meeting point and grotto are places where i got mine

grotto has a lot of other bronzes and will give you very few, though. because of that merlins and goukiris might be better (but i cant confirm that because i just identified a good 1k bronze weapons i had stored away from grotto ...)
(i have 2 spare monster wolf tachis if ya wanna buy >,> ... grade is only 47 and 6x though)

the low level tachi for the forge can be found in byakkou caverns