Ruger makes 45 ACP cylinders in two sizes, mid frame (like yours) and large frame. In most cases such as yours, it's far easier to sell your gun and buy the gun you want since you aren't familiar with doing this. Much less hassle as well.
Plus, stainless mid size 45 ACP cylinders are new and hardly available. If you want to search for one you need an overall measurement of yours in .001". Buy one no shorter than that.
Or get one here for $250: http://www.reedercustomguns.com/information/services.htm
Just swapping guns is probably starting to look more appealing....