I work at a veterinary hosptial, so I get heartworm and flea meds for free as well as meds at lower cost, but.....
We get a lot of prescriptions from 1-800-petmeds and petcarerx. Often to help save clients money the Dr.s will write a script and recommend they take it to a human pharmacy that offers $4 generics like Target etc... they will fill them for pets at the same price. Kmart even has a specific pet RX discount program and can provide medications that are not also for humans.
Regarding the inability to guarantee Heartgard purchased from any source other than your veterinary clinic..... Merial, the pharmacutical company that makes Heartgard guarantees their product to be affective, but only when purchased from your veterinarian and it's given according to his/her instructions. So if you dog is infected with heartworm while following the prescription, Merial will pay for the treatment. However, they will not if you purchased it through any other sources like petmeds or petcarerx. It doesn't mean it's "fake" but because they cannot guarantee the product didn't go directly from their hands to your vet and is genuine and/or not relabled generic products, they will not. It's in all likelyhood exactly the same product but I guess you can understand that they cannot control who distributes it or handles it so they won't guarantee it. Some clinics like to go a little overboard on giving you this warning but we just make it clear it's only guaranteed if you buy it from us. Otherwise, we're just happy they put their dogs on heartworm preventative at all. (I work in a low income area).
BTW, there is nothing wrong with generic heartgard either. Iverhart has been around for a while. Any drug with the active ingredient ivermectin will prevent heartworm. Generic ivermectin is available without a prescription, but it's best you know what you are doing to dose correctly before trying that.
Frontline is different. It's a non-prescription medication. You can shop online for the best price from any source. I have seen it on sale a lot at drugstore.com for a better price than we sell it for. I also trust the companies Revival Animal and KV Vet. I have ordered lots of products including vaccines from those 2 companies. Both of these companies also sell to veterinary clinics, but anyone can order non-prescription products from them, as well as provide a signed Rx for anything that requires one. Vaccines are restricted (or not) according to your specific state laws.
regards