They've been discontinued for several years now, but you can still find Uncle Mike boot grips for the RB j-frame occasionally on ebay. S&W apparently has quite a stash of them, I bought some a couple months ago from them for about $22 delivered. I've never heard of or seen a set of UM boots for a SB j-frame, and I looked. Ended up buying a set of boot grips for my SB model 36 that Jim Badger made up special for me instead.
I also bought some Badger boots for my RB 36 -- his standard boot grip extends below the grip frame but (again) he made these up special for me with an open butt. I really like them but would like to try some others.
Tried a (used) set of Ahrends but they're a bit too slim, I prefer some palm swell. Thinking about trying Altamonts or Spegels. The uncle mikes were designed by and/or based on Craig Spegel's design(s)-- are the genuine Spegels pretty much the same shape & profile? I like the uncle mikes shape, maybe even better than the Badgers, but would prefer wood. See the thumbnail for the difference in the finger-groove area between the UM's & the Badgers. This photo doesn't show it but the uncle mikes boots don't fill in the area behind the trigger guard as much as the badgers.
Any feedback on either Altamont or Spegel RB boots re: shape would be appreciated.