Daryl
I seriously doubt you're going to find many, if any, of those "cartridges" in the wild. Your best bet, would be to find just "one" and then find ya a crazy machinist to make you several more. You need to talk to your Broker, preferably over a good lunch, before you embark. It ain't gonna be cheap, Trust Me.
And, if you happen to have a Roper Revolver, you're gonna give the "Collector" apoplexy at the thought of actually shooting it. Were it mine and were I crazy enough to shoot it, I would first take it apart and duplicate every single internal part.