Best place to buy prescription pet food?

My cat Pippin just got an Rx for Royal Canin SO and will need to eat this exclusively from now on. I bought a 7.7 lb bag of dry food and two cans, and it was $42 at the vet. I'm hoping maybe some of you know of a good online vendor where maybe I can buy in bulk to save some money. Any suggestions?

Thanks for any help!

I found numerous places on line when I first started having to give Myndi prescription dog food for her kidneys. I believe they all require a prescription. I just buy it from my vet, since it's more convenient and there's really no difference in price. Actually I found my vet to be cheaper than most places.