I am so sorry to hear this. I worked in a vet clinic, and have never seen a clinic do discounts on heartworm treatment or other severe health problems, I even called around for our patients. Some can set you up with a pet health credit card though, that will allow you to make monthly payments. You will have to call around and ask who offers this.
Treatment can be a long and expensive process, depending on the degree of infestation. Your dog has had heartworms for at least 6 months, because before than, it would not be able to show up on a test.
Your dog is 9 years old, so if he is in good health for his age, he may be able to withstand treatment. (Smaller dogs tend to age better than larger dogs).
However, if he already has health problems, the treatment can cause a greater risk of fatality. To not treat him, he would die of heart failure, not a particularly easy death. So, if this is the case and if you are unable to treat any time soon, and won't be able to in the future, it may be more humane to euthanize him.
Here are some links that can give you more information.
I have heard there are natural rememdies, but I don't know a thing about them. Try researching it and seeing if it's an option.
***Oh, and a note about 1-800-PetMeds, due to some bad activity many years ago that they have cleared up (selling international repackaged expired medications), MANY vets will not approve of prescriptions to be purchased through them. PetMeds has to call your vet for prescription approval so they can fill your order. So, if you need to use them, make sure you have a vet that will approve first hand.