Getting the medication shouldn't be hard. Just call a vet office, see if you can speak to the DVM, and worse case they make you make an appointment, but more than likely they'll just write you a script once you explain to them what it's for.
The only reason it's controlled is so dumb people who let their pets go without it for years, developing heartworms, don't instantly kill their dog by giving them the medication and it quickly killing all of the heartworms, leaving bleeding holes in the animal's heart.
Vets get that, and once you explain why you need it/what it's for, then it shouldn't be hard to get it.
You could also look at the Beyer Advanced garden pest killer. There are a lot of threads in the SPS forum about using it as an alternative to interceptor since it was off the market for so long.