In revolvers you have to start out with a cylinder and ejector ASSEMBLY. That is, a cylinder that's had an ejector fitted to it.
S&W, like most gun companies will not sell a new assembly. For safety reasons, they will factory fit new assemblies.
To fit the cylinder to the frame is a MAJOR operation, with a lot of adjustments and checks that have to be made.
Note that cylinders and ejectors will rarely properly fit another gun, since they've already BEEN fitted to a different frame.
Among the fitting/adjustments/checks are:
Head space.
Barrel/cylinder gap.
Barrel/chamber alignment.
Timing.
End shake.
The Kuhnhausen book goes into some detail about how to fit a new cylinder assembly the RIGHT way.
Don't do it the right way, and the gun will range from inaccurate to unsafe to fire.