I've been seeing this drop off of the random rat men monsters in Dragon Necropolis. I've seen it drop from "an escaped worker" and "a Paebala rebel" for certain, in the Paebala cave. The Batfang Headband has dropped from "a Paebala Spirit Talker" for me as well
Update: I've also found out that they drop off of the Chetari in DN as well; so far getting one off of "a Chetari guard"
Edited, Jun 16th 2018 8:10pm by Uprise