As if prescription food wasn't enough, Royal Canine has introduced prescription treats. I didn't check the ingredients (and really don't want to), but if your cat is on prescription food, you can now get matching urinary treats, gastrointestinal treats, hydrolyzed protein treats for cats with food sensitivities, and satiety treats for cats who are on weight-loss food.All of these are available only from your veterinarian, of course.
But if you're willing to do some research, you can find food that meets your cat's special needs on most pet supply retailers' shelves.
For instance, several cat food manufacturers make wet food with novel proteins for cats with food sensitivities. And the best diet for cats with bladder stones and crystals is all wet food and no dry, even if the wet food is something as simple as Fancy Feast.