Hi FlanneryCam I can totally relate my little guy, Bandit has IBD - Inflammatory Bowel disease and I find it so hard for him to find a food which he can eat that doesn't make his health worse.
I would try Felidae Senior food for a wet food, and combine it with a dry food. This one has very very low fat content (making it easier to digest for older cats) and is grain free too so you don't need to worry about it upsetting your cats tummy. I totally recommend it. It is much cheaper than other fancy cat foods.
Another tip is that crystals often form due to hard water, so if your town has especially mineral rich water you should either buy bottled water, or boil the water first before you give it to your cat. This will reduce the chances of your cat.
This is what my guys get daily:
1/2 can of Felidae Senior mixed with some filtered water
1 cup of Lifetime Cat, Turkey and Oatmeal dry food.
Bowl of Filtered water.
It may sound silly but changing the water can really help, and although the food I have suggested sounds fancy its pretty cheap. The large can of the Felidae is $2.79 and it does 2 cats, 2 days, and the bag of dry food is around $12 for 2.5kg and it lasts 2 cats around 1 month.
Here is the information on the wet food:
http://www.canidae.com/cats/platinum/canned.html
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