You should be buying your medications directly from your vet. Only they can give an exact diagnoses as well as the best way to treat any illness.
My own vet gives me more medication than I need at one time in case I need to continue treatment for a longer period, or for those times when the clinic isn't open and I need something to hold them over until I can get them in. She and I have a good relationship, and she trusts my judgment in treating my rats. She also knows that if I have to start them on medication for any reason without her, I'll be in soon enough to go over everything with her and see what she has to say about the situation.