Hi, Had quite a few of these drop off yard trash in the desert area in Plane of Storms. Can't recall specific mobs, we were killing dillos, cacti, scorps, snakes, rock storms, and a few giants. In just a few hours we got enough for everyone (full group) to get 6 apiece. I also recall these dropping in Nightmare I believe. May be common throughout PoP.
This is used in the new Planar armour quests. Two strands of this and an ethereal curing agent make an ethereal silk swatch. This requires a skill in tailoring of 200.
It actually *Requires a modified skill* of 220. So 210 with a Geerlock will work. Anything lower gets you a message that you are not high enough skill to attempt this combine.