I found nothing for 15T servo savers. For the uses of these servos, for like diff lock or 2 speed, servo saver isn't really needed. The savers (mostly plastic ones) make it sloppy. And sometimes for it being sloppy, it might not steer properly so whatever its shifting, it might not actually...
so are you trying to use a 1/10 servo horn on a micro servo?
Idk of any adapters. This also depends on how many teeth the spline has.
I did seen servo horn adapters. I saw them when i got a traxxas 2wd servo saver replacement. Comes with little plastic adapters. Not sure it is what your looking...