I placed an order for Frontline flea preventative from EntirelyPets and received it very fast. I was able to access online help, because I was looking for Heartgard but couldn't find it on their site. The reason I couldn't find it on their site is because Heartgard is a prescription pet medication. So, if you are looking for a place where you can buy your general pet supplies and your pet's prescriptions, this isn't it. However, they were very helpful, the price on Frontline was good, and the order arrived promptly and in good condition.
I ordered the Heartgard from 1-800-Pet-Meds, but I need to mail in the prescription from the vet to them (1-800-Pet-Meds accepts the okay over the phone, but my dog's veterinarian won't do that, for whatever reason.) It's a real pain, but it's not 1-800-Pet-Med's fault.
You can access both EntirelyPets and 1-800-Pet-Meds through ebates to save even more.
Currently ebates is offering 8% cash back on 1-800-Pet-Med orders and 3% on EntirelyPets orders.