Check into the caresheets and threads on appropiate sized enclosures for ball pythons as well. "Getting a larger cage" is not always a good thing, so please check into that.
A pet as a impulse item is not a good thing. It's sad that you trusted someone that SHOULD have given you good advice, but instead chose to give you poor husbandry advice, and seems to not be concerned that they are selling ill animals.
I'd insist on a immediate refund for the purchase price. You will not get a compensation on the vet bill. That was your choice(even if it was the correct choice, unfortunately). Then make sure your first python has a correct enclosure, and then think about getting a second one from a more reliable source with a second enclosure.

Good luck, and welcome to keeping herps.