zcb, you don't want to free-lance heartworm treatment. If the cat has a mature heartworm, and you kill the heartworm, you risk killing the cat. That's because heartworms aren't adapted to the cat's circulatory system. They're too large and a dead heartworm may very well cause a blockage. It's generally safe to risk up until six months after the introduction of the heartworm larvae; after that, the only safe treatment is symptomatic treatment. Thus the need for a prescription and the demand for bloodwork.
The heartworm cannot reproduce in a cat, so it eventually dies a natural death, and that's less dangerous. If you want to wing it, however, you can probably find someplace outside the country that will sell Heargard online without a prescription. Just be aware of the risks.
Heartworm in a cat is best dealt with by prevention, not by treatment, so you concern is well-placed.