From other posts on the forum, S&W doesn't have the LR cylinder any longer. You would have to find a LR cylinder and have it fitted to you Mod 48. It may be easier to buy another revolver in 22LR than to locate the LR cylinder. I don't have the extra cylinder for my Mod 48 but do have a Mod 18, both are the same 4' barrel K frame and handle the same, one in 22mag and one in 22lr.
Good luck in your search.