Is there a list anywhere for where they can be found/purchased? Im in need of vithrack brains and troll skin late game. Party lvl 16

No idea about a list. But Vithrack Brains 100%: Splintered Reef, tavern barkeeper.

 

There are also some in Nekataka but I can't remember where.

Cuitztli's Exotic Herbs in Periki's Overlook in Neketaka (randomly, may need to wait 1 or 2 daily stock refreshes). I think The Dark Cupboard in that district can also roll some on stock, not entirely sure though.