Dixie gun works has a shoulder stock for Pietta 1858 remingtons, and their freak buntline colt navy. It costs about $150. You could get either a normal steel framed remington, or a steel framed remington "buffalo" which isn't historically accurate, and just add the stock when you felt like having a carbine. During the civil war a lot of higher end pistol sets came with an add on carbine stock. It might be a good alternative if you didn't want to pay for both the pistol and carbine.
Ooh, I just realised, you couldn't shoot a conversion cylinder with the carbine stock on a pistol, it would be illegal. You could use the cap and ball cylinder though. You would need the Cattleman's Carbine with a 16" barrel to legally use a conversion cylinder with a stocked gun.