Heartworm medicine without a prescription

[dkohli0000]

I've been looking at websites that let you order heartworm medicines without a prescription.  Has anyone had any experiences, good or bad with this?  These websites offer both heartgard and other generic medicines.  I'm thinking about ordering from one of the websites but I don't want to get ripped off or get something that would not work on my dogs.  I would greatly appreciate any advice.  I've thought about going to the farm supply store and getting ivermectin for horses or pigs but I'm nervous about doing that, I probably would not get the dosage correct for using it on the dogs.