I don't think that friction grip shifters are available off the shelf. You could probably make one, though. If you take apart a grip shifter, there is usually a metal spring that is used by the indexing mechanism, you can just pull it out. Once you have the indexing disabled, you need to figure out a way to hold the shifter in place against the cable tension. One way would be to cut a horizontal slice partway through the outer part of the shifter and put a hose clamp around it to adjust the friction. It would be ugly, but it should work.