What is a good food for dogs with urinary issues?

My dad has a dog that has been diagnosed with kidney stones and a UTI. She is a 9 year old spayed female rottweiler. I am not sure which kind of stones (struvite or oxalate) she has but she was treated with an antibiotic for the UTI and given a prescription food (i think it was royal canin urinary health) by the vet. My dad however can't really afford the prescription food and I have been looking into buying some food for the dog, but being well aware that many of these prescription diets,well may help regulate certain levels that help with urinary health, still use crap like by-products and corn. I want to find a natural food (with the quality of such foods as natural balance, blue buffalo, etc) that will also help with her urinary health and that I can get over the counter for him. Does such a food exist? If not does anyone know the levels of things to stay away from in foods so that maybe I can find a food that would still keep her healthy but be of better quality? Thanks for the help! I definitely need a point in the right direction.