In the underdark for sure, a dwarf by the Myconids. She sells ingredients for about every potion/elixir available for your level. If go in from the Zhentarim route or the Selunite temple, you only have one fight on the way. She also sells randomly rotating stuff, and, I think, fixed healing supplies.
There's two vendours in the mountain pass, but I'm not sure if you'll keep them alive. The underdark dwarf can move with you into Act 3. Maxing her approval is not a full waste in that sense. Only give her the noblestalk from her quest if you want her to leave.
There's also some pretty great boots by a wounded gnome near the myconid king, getting those boots alone would be worth the journey. (You don't have to kill her for it)
