I was in Petsmart the other day and I saw heartworm medication, with the label, for vetinary use only. Don't know why they're selling it. But you can purchase it and show it to your Vet.
I order my heartworm meds online, which saves money, but I still have to have my dog checked at the vet before they will send any meds out.
Also, Check out Care Credit.Com. You may qualify for their services...It's a Low Income Credit Card used specifically for Animal & Human medical expenses...You can apply online...It wouldn't hurt! They are very low interest & alot of Vet Clinics and Hospitals accept it as a form of payment......Good luck....Karla