Calcium oxylate stones develop in acidic urine and struvite crystals develop in alkaline urine, so you want to keep the urine in a 5.5 to 6.5 pH range. The easiest way to do that is to feed fresh meat. Cooked or raw chicken, with its bones, along with some cod liver oil and plain yogurt is probably the cheapest and easiest way to feed a cat. Be careful about adding any supplements, as they may change the urinary pH.