The above mentioned links are great.
You can also get a free diet from a website that is run by a vet nutritionist. For a pup that has medical issues, you just submit the vet's contact info and they will get approval. From there, you will be able to choose a diet based on the restrictions that the pup needs. You will need to purchase the supplement, but you need them for any diet formulated by a vet nutritionist.
www.balanceit.com