The yoke is a factory fitted part. S&W usually refuses to sell critical parts like these.
Among other areas to be fitted, the yoke has to be ground and polished to match the front of the frame, the cylinder assembly has to be fitted to the yoke to insure proper head space, end shake, alignment, timing, etc.
You could pick something up on the internet or at a gun show, but expect that the yoke probably won't match the frame, the cylinder may no longer fit the frame right, and that the yoke may well be bent or damaged, which is why these usually turn up for sale.
Best option, send the gun to S&W for a properly fitted part.
