The prescription foods are quite expensive. Ask your vet about them cause there are several. There is one by Hill's called S/D Feline and there is one by Royal Canin called URINARY S/O. In My opinion out of these 2 I would use the Royal Canin as it deals with both types of crystals whereas the Hill's only deals with one and can sometimes cause the others.

There are also others so like I said give your vet a call and find out what types are available where you live. If you tell me the country you live in I can get you the price on the Royal Canin food.

Michelle