The service folks at Ruger will supply and install a new convertible cyl for $185 including return shipping. They will not just sell you a cylinder.
It's a little expensive to send your gun to Ruger however so you'll have at least $280 in total cost including shipping your gun to Ruger.
Cyls are available in this forum's classifieds, e-bay or Gunbroker among other places. Price range $65 to $100 for .22 depending on condition and seller.
Here's the problem sending a Ruger back to the factory for anything, the shipping cost is the killer. About $100 for an individual to ship them the gun and $30 return shipping. But having an unconverted old model is a huge advantage! Here's what you can do: call Ruger (336) 949-5200 and request a transfer bar safety upgrade which is free INCLUDING shipping both ways. They'll email you a free shipping label. Request replacement of the missing cyl, refinish, or whatever while they have it.
Even if you don't want the gun converted but here's the deal: the conversion does not modify your gun in any way, only the action parts are replaced and the old parts returned to you. You can even remove all the action parts including hammer and trigger before sending it.
When your gun is returned, simply remove the retrofit safety parts and reinstall the original action parts. The bottom of the frame under the trigger guard is usually stamped with an "R" which is not visible when the gun is assembled.
But you just saved $130 and got a new cylinder factory fitted for $150 instead of about $280.
ps
