What food should i use for urinary problems for a male cat?

My cat Lionel is ten years old and still active. However he has had reoccurring urinary tract problems. I recently took him to the vet and they gave me a cat food (Prescription Diet C/D) which Lionel is supposed to eat for the rest of his life. (It has low magnesium). My problem is that the food is very expensive and I have to go to the vet everytime to get it. Is there another kind of dry cat food that is available? I heard that Nutro is a good brand. Also, I would like suggestions on any alternatives beside the dry food. I can ask my vet these questions but I would like to have a 2nd (and 3rd, 4th etc) opinions from Yahoo answers. Any advice would be appreciated. Thanks!