As far as I know, the cylinder has to be timed to the gun, you just can't order one and drop it in. If it didn't come with the gun, it has to be custom made.
Not true in every case. OAL is the key...Keep an eye on the RugerForum.com and you may get lucky with one. Many will drop right in, Rugers are nice like that. I have had luck in the 22magnum old and new models and the 45 Colt. Never had to fit any of them and none spit lead. I use the 45LC cylinder most of the time anyway, just load it mild to wild.