There's a few ways this can be balanced. The mana pool could be increased, the spell cost cout be reduced (which is basically the same thing). Mana could regenerate over time (very, very slowly would be my vote) or encounters can be reduced or spread out.
My own feelings about this aren't particularly set in stone, because not having played the game it's tough to get a sense about how it all fits together. In general, I would favor potions not littering the game world like a typical ARPG, instead maybe just make more food items that restore moderate amounts of health and mana, particularly early in the game.
