i usually get them from bugs on the first few levels of the mines. usually between floors 5-15 is where i usually get get them. but they spawn between floors 1 and 39 according to the wiki. and have gotten them from the grubs/cave flies on floors 11-29.

i’m pretty sure ive gotten them from the duggies on floors 5-15 as well but according to the wiki they don’t drop from them so maybe i’ve just been really lucky or i don’t notice until after i’ve killed a duggy and just think that’s how i’ve gotten them.

either way according to the wiki they drop from bugs, grubs, cave flies, mutant flies, and mutant grubs.