I am sure Dr. Doug will chime in shortly but I believe a vet has to purchase all his medications from a pharmaceutical distributor. They are not allowed to go to the corner store and buy medications for their clinic. IMHO too many vets get a bad rap about med prices and it's usually because the client is not aware that he has to purchase them and pay whatever the distributor prices are to him. They have no control over that nor the availability of meds.
Vets most certainly can offer a client a 'prescription' for meds if the client feels he can obtain them for less at the drug store. I've never met a vet that would not offer that option.