Is there a way to purchase great balls or ultra balls? I'm on level 21 and these have been unlocked but I'm about to make my first purchase in the store and wanted to know if I could only get pokeballs?
At this time you can't buy great balls or ultra balls. Besides that I highly recommend you spending the money on lucky eggs instead of poke balls, since poke balls aren't that rare.
There are currently two ways to obatin great and ultra balls:
1) Unlocking them at the given level (12 for great balls and 20 fors ultra balls) and thereafter as a reward for certain level ups
2) After having hit above mentioned levels, by getting lucky spinning a PokéStop.