Is there a less expensive way to buy flea meds and heartworm treatment other than paying through the veterinarian? Do you have to have a prescription from the dr?

The Advantage is best in the long run. Anything the stores sell is junk and you end up spending more. Zema and Zodiac are better than Hartz and Rid but they still aren't that great.
Petsmart has an inexpensive vet clinic Banfield
PetSmart.com: Banfield

I buy on line the revolution at Pet Shed .com they re more reasonable than the vets are.

You will need a prescription for the heartworm medicine. You also need to do yearly heartworm tests, since you can't use some of the preventatives if there is an active infection.