I've never done this but if I did, I would use a Canadian pharmacy. Google the pharmacy name for reviews and comments before actually placing an order. I have avoided a few bad online places by doing exactly that.
Edit: I started looking online, and it looks as if the country (such as Canada) requires a prescription, you still need a prescription to order online. Apparently Mexico does not require a prescription; however, I think there is more risk of getting fake medication with that route.
I did find this article online:
http://www.coreynahman.com/antibiotics.html
This is an interesting question!