Spoiler warning: Quest and/or game spoiling details follow.
A lot of them can be found at daily spawns in Edron Goblin Cave, west of the triple Hunter spawn near Edron and in Hell Gate in the elf area. There also spawn 14 of them in the War Wolf Den on Rookgaard. One is also required to get access to Talahu and then also Malada for The Shattered Isles Quest.
Spoiler ends here.