I agree that if the vet told you it was okay, then it should be. I know there's one that says DO NOT use on cats and I think it's the Frontline. I would have to look at my box at home. Sorry about the Capstar situation. Maybe if he would give you a script for it, then you could order some from PetMeds or somewhere.
Okay, found this for you.....
Capstar - 1800PetMeds.com
The price is quite reasonable compared to the 5 bucks I pay for each at my vet. I don't know how petmeds.com works though. I think you have to have a prescription for some things. I'm sure someone else can help you out on that. There's some reviews to read too regarding the Capstar. There's Capstar for kitties too.