There are potion vendors - e.g. there's one in the villiage at the start of Broken Valley - near the middle, out in the open air (you can see the table covered in potions). I think there was also one in the bandit camp, but I'm not sure.
Later (after the temple) you'll get your own alchemist as well as a servant who collects ingredients for you, so you'll become self-sufficient.
Your level might be a little low - I was level 15 before I entered the temple, but it shouldn't be too bad. Try dropping the difficulty to "casual" (if you haven't already done so), and keep your creature summoned at all times. If this doesn't work, try going back to Broken Valley and gaining a couple more levels before trying the temple again. And put one skill point into the healing spell - it helps you conserve your potions.
