Or you can build one if you happen to have a Model 17 or 18 handy. See the thread on the gunsmithing forum
Cylinder Fitment The cylinder is available from the 648 stainless .22Mag from Numrich for about $75 and it seems to fit the yoke without much work on the 17/18's. Fitting the extractor is the hard part but most gunsmiths do this easily. Here's mine on a pre-18 chassis.
Stu