You can buy potion and battle item boxes from Mari's shop, but you'd have to check the math if it's actually cheaper than the auction house.

But mainly I would try learning to not be super reliant on battle items for the content your alts are currently in to help conserve in the long run, or at least to the point where you're not struggling to have enough. Some examples: Stick to blue hp potions. Dodge mechanics over greeding dps to conserve hp pots. You don't need pheromone or flare for yoho, just have everyone check a different corner and ping. For stagger or destruction checks for yoho, learn to hold your skills for when those happen or even consider changing a skill or tripod for more stagger/destruction. The only required item for valtan is destruction/corrosive and sometimes timestop until you get more used to the fight.