I use Google shopping - put in the medication I want by name (in our case, Soresto collars), then choose the place with the he lowest cost+shipping. This year, that was Walmart; last it was something like PetMeds. XL collars run around $45 each and last 8 months, which gets us through flea season. My heartworm treatment comes from the farm supply store, which you should not try unless you have very large animals already proven to be unharmed by Ivermectin and you know what you're doing in terms of dosage.