when Seamus was so ill, our vet recommended that we fill his prescriptions at CVS, WALMART, etc. he didn't bat an eye. In fact his penicillin was free at Shoprite where they run a special all the time. Shoprite is an east coast grocery store in the US.
My vet did however control how much of the drug I was able to purchase at one time. It was kind of a skimpy amount, I don't know why except maybe he was watching out for us because Seamus' illness was so severe and he didn't want us to have a backlog of meds.
I checked for you and they have a couponmom in Canada. There are several sites and some good information.
Kate