Contact Ruger and see how much they want to install one. May not be the cheapest route, but it will be done right. I don't know if Brownells or Numrich might have any 22LR cylinders or not, but you can look.
http://www.brownells.com
http://www.e-gunparts.com/
http://www.brownells.com
http://www.e-gunparts.com/