I got mine from Vet-Medic but it was a pack of 136, vets don't like prescribing for more than 3 months unless you can convince them that is the pack size.
Just for info, when I asked my vet for a prescription he agreed to drop the price by 15%, still not as cheap as Vet medic but I don't have to pay for the prescription and can buy in smaller quantities which I prefer.