You only need a prescription for meds that would normally require it. The flea meds I use doesn't require one, but if I was to get the Heartguard on line, I would need a prescription. The web site will indicate if a script is needed or not.
If your vet is cool, they can fax it, otherwise, just get a copy and fax it or mail it yourself.

The site I buy from is: vetshoponline.com