None that are cheap.

Mamiya makes a grip for the RB and RZ that is comfortable and works fine - new in the neighborhood of $200. I checked KEH and no used ones at the moment. They have 2 pins on the bottom that fit in the bottom of the camera so the grip won't rotate. Either the RZ version or the RB version will work fine - the RZ version has a trigger button on the grip but that won't work on the RB since the little cord has no place to go, but otherwise is ok.