I myself have had both a urologist and a nephrologist. The bad news is that nephrologists generally only deal with patients that have some type of history of kidney disease. That means that unless you have had at least a kidney blockage from your stone(s), they would have you see a urologist.
With that being said, if you only have one urologist around you, you could always try a nephrologist's office and ask if they will see you. The worse they can say is no.
Good luck to you and I sincerely wish you good health!