Is there any guide or list out there for where to find the various potion reagents? Most of the high end stuff is pretty straightfoward, but I can't remember where to find Dragon's Eye other than Eofol, and I do remember there being quite a few earlier in the game.

Clanker's Lab (MM7) has a number of ingredients

The stronger reagents might spawn as mid or high tier treasure.
They may be carried by the better magic item shops and alchemist shops.
Clanker's lab is the best source of a lot of reagents.
The ore smith in Erathia that makes items might make one from the stronger ores.
A dragon might drop a dragon eye when killed.
Demons might drop ichor when killed.
Elvish Toadstools can be looted from Avlee and the Tularean Forest.