There is only one zone and one mob type I know of that drop these in COV. There were at least two zones with 1 mob type in TOV. After hunting on multiple characters, doing hunter achievements, collections, 3 earrings, etc. After a year, I had a little over 200 of these saved up. It would be fantastic if it could be added to more mobs loot table. In EOK/ROS it was sarnaks/humanoid which were very abundant. I think it has been on humanoids before that too, but I didn't pay attention so much before EOK when I made my rogue.