I got lucky and had an easy time with Brindled since it dropped extremely commonly from crocs, though their loot table has since changed unfortunately.
Otherwise I've gotten everything else maxed out now, with the exception of Black Silver... been trying since hitting level 20, and easily gathered 1k+ silver by now but no luck so far.
I do think level might play a factor though, at least from my experience with Silk. Before I was up to that quest I was getting it to drop fairly regularly. I didn't end up picking up the quest for it though until the mobs were gray-cons for me, so had to do a bit of backtracking to get the 20 to turn in. I spent many, many hours clearing out the Cannibal Cave in Eggo Mountains to finally get all of them ... so yea, at least for me it seemed my level played a role in drop rates. :/
Now I just hope the bugs with the 'alchemy kits' are worked out soon so that I can finally move beyond T1 with it ... assuming I can get them to drop from gray-con mobs that is