For the OP; Your question on adding a spare cylinder in .45 Colt.
IN GENERAL,, yes,, it's actually easy. You measure the existing cylinder from your gun, for the overall length. (OAL) then, you look for a Ruger built, .45 Colt cylinder, with the same OAL, or even a few thousands longer. If it's a little longer,, you can gently hone the cylinder boss to fit it to your gun.
As for timing,, it's not very often that a Ruger built cylinder won't time properly. HOWEVER,,, you should ALWAYS properly check the bore to barrel alignment PRIOR to shooting.
Many feel the SA market is declining,, and in some aspects,, it is.
But there's still a very viable market in general. Otherwise,, the prices would be lower. High prices point to a strong demand.