Abby gets those too sometimes. I bring her to the vet and they stick it with a needle to drain it and the first time gave her Simplicef. I know that Walgreens doesn't carry Simplicef but they DO carry the generic version Cephalexin.
I actually found a pdf online telling vets how to "fight for your business" and learn to prescribe things that regular pharmacies don't carry. I don't know if I'm allowed to post that url here tho. It's very interesting reading.
I can PM it to you if you like.