i have no recommendations, but does your vet support natural remedies, you could ask for a recommendation from your vet of what would be safe to use as an alternative or with the current treatments. Please use caution when choosing natural products. natural does not equate to safe or better. they can be but not always. im sure people here will have some suggestions but before starting any additional treatments consult a vetriarian.
Since kidney stones are likely to be recurring a good preservative free natural diet will help prevent more (which youve said you've switched foods, what have you switched to?) and make sure to always keep lots and lots of fresh water available