You need to get heartworm medication or at least the prescription through your vet. It is not available over the counter. Dogs must be tested yearly to make sure they do not have heartworms because giving them heartworm medication if they are positive can be dangerous. It kills adult heartworms which can block the arteries in the heart and kill the dog.
Flea medication is over the counter. There are a lot of counterfeits on the market so make sure you buy it from a reputable company. I get mine from Entirely Pets.
Counterfeit Pesticide Products for Dogs and Cats | Pesticides | US EPA
My personal preferences are Interceptor and Advantage. If your dog has seizures, it's important to avoid Heartguard and other heartworm medication that contain Ivermectin as studies have shown it can lower the seizure threshold.