It is good that they are already on Royal Canin, yes mine is prescription only available from the vet too, but especially for cats with urinary problems, so if they had to change to that, it would not be too much of a problem as they are already used to the taste, took a while for my lexie to adapt to it, but she is sweet now.Good luck.

Ok i had not read your post before i typed this,i find it odd your vet would not strongly recommend that you put them on special food for the problem,or have i mis-read your post, first thing my vet did was change to the royal canin SO ,it has to be that kind not just the royal canin food,which is specifically for that problem, well struvite crystals anyhow, and i think there is one for the other type of stones as well.