You need a vet to approve a prescription in the U. S.
I’ve been getting mine from Canadian Pharmacy since I got Buddie eight years ago. As of right now, you don’t need a script but I usually get one when I have her in there for a checkup. I then fax it to CP.
When I started, I paid for the free shipping for life and it paid for itself years ago.
I get their generic and it’s less than $20 for six months. As I said, I’ve got it there for the last eight years and every year Buddie has tested negative so their generic works---guess it should since it’s a generic for Heartgard.