My Ruger Single Six only came with the 22lr cylinder.Just check out all the common gun auction sites and click on to the gun parts section then use the key words"Ruger Cylinder" and pick one out.It worked for me and dropped right in.
The Lr will fire in that cylinder, but will swell and be harder to eject. The forcing cone will center the bullet ok where I didn't feel any splatter. Not a good practice tho. Much easier to find the LR cylinder than a mag cyl.