From zone line head NW to the green wisps, treants and mulchs. The viridian ghosts do stun, mez and mem blur but have the best drop rate. Treants do snare but their drop rate seems good too. Stay away from the Kirathas village, after about 100 kills there still was no drop. You might also want to farm for the "Dark Black Gem" augment (20 AC, 45 HP) which is a zonewide random drop.