I think you should have a word with the vet about his poor receptionist, I can't believe she doesn't know about the prescription foods - mine have a booklet hanging up about them all, it takes seconds to check. My vets always enter prescription food onto their records, and they check them before dispensing them. If you buy prescription food online, do be aware that you get charged p & p, last time I worked it out, it didn't save me any money.
I found the same probs as Helen wiht finding baby food - in America, it is easy, not so over here!!