I have the exact same problem with my cat, she's been clear for about 5 years now, but she HAS to stay oh her own special food,or the bladder stones come back. I started noticing there was a problem when she wasn't moving around much, and spend soooo much time on the toilet not actually peeing, but trying. (ya, she pees in the toilet, don't ask me how she learned that) And then she started peeing on soft things like my bed, the couch etc. Anyways, she was given these antibiotics, and some special food and we got lucky, and the stones disolved. Now it's more complicated because we have two cats, and we have to supervise suppertime because each one likes the other one's food, lol!
Anyway, I feed my cat Royal Canin Medi-Cal urinary S/O. In the green bag. It's only available from the vet, but it has a little maple leaf on the bag, so I'm not sure if it's available in the US. It's also available in wet food, so that's a plus, my cat thinks she's getting a treat :)