I wouldn't buy anything for my pets that they would ingest from anyone I didn't know, regardless of what it is... food, medication, treats... I don't trust people enough to let them give my dog or cat any kind of food product.
You can get an RX from your vet and buy Heartgard from
http://1800petmeds.com or other sites but, usually, your vet will have comparable prices AND I usually get a rebate if I buy Cooper's Heartgard for the entire year.
Please don't buy anything for your pet that he or she will ingest from someone you don't know... you never know what they have done to the food/medication before it gets to your house.