Bz,
The problem is that you will not be able to purchase heart-worm medication for dogs and certain vaccines (for dogs or horses) without a prescription. The reason being is that with heart-worm medication, if the dog has heart-worms and you administer it, your dog will die (more than likely anyways).
Any online (Canadian or USA) sources will have to call your veterinarian to request a prescription. I know this because they called the office that I worked for on a daily basis, and every time we would just reply that they need to have an exam or a heart worm test before the prescription could be written.
As a side note (it is obvious I have a bias in this matter), online pharmacies used to contact us on a monthly basis in an attempt to purchase our OUT OF DATE / EXPIRED medications so that they could resell them online. That in my opinion, is a red flag. In all honesty, the medication would probably still be just fine, but I am not so sure that it is worth it.
Sorry that I couldn't be much help. I am sure you could get the medication at a much cheaper price from a Canadian source, but I am certain that you would still need a prescription for the heart-worm medication.
