I never had the patience to camp merchants.

I'd just check out merchants for mixable potions when I was in the vicinity, then mix up what I could and sell any results I didn't need for my party's own use. Any potions I didn't have anything to combine with I just lugged around until I found whatever I needed to mix them with.
The character with the lowest skill would mix what they could, then the next higher skill would mix what they could, then the character with the highest skill would mix the most difficult ones.
That, plus spell casting, was enough for my ranger, alchemist, and bishop to easily keep their Alchemy skill high enough to get new spells as soon as their character level was high enough.