Yup plenty of "R" kits for sale. I just found a dozen or more on eBay. However I should elaborate by stating these are usually not a conversion kit, but a modern "emulator" kit that performs as an R without any machining required. They are very good I'm told. However I have found several new piston / rebuild kits for original R servos, for which you obviously need the OEM cover and I guess a CJ iron tailed C6.
However a modern rebuild guide book I have and several online magazine articles on building C6's all recommend fitting an R servo, with one having the proviso that machining "may" be required. Also my local shop mentioned there is a guy here in Melbourne who does the machining to take the bigger OEM R servo, for guys building a CJ that want the right cover when the original transmission has gone missing, but added that they themselves usually just fit an R "emulator" kits, as they perform just as well and pop straight in.
So the word "commonplace" might not be the best choice. Likewise when the C6 builders selling uprated street / strip type transmissions state they have an R servo, I'm imagining they have simply fitted the emulator kit rather than machining for the big R servo. I mean, why would you bother, if it's just performance you are chasing?
Sounds like the H is a good option for someone looking for a period upgrade...