have never seen an original for sale, but i have seen the patents for them online and wanted to make my own.
for those that dont have one, its basically a kit consisting of a replacement cylinder for 22 rimfire cartridges, a loading gate/conversion ring, a special barrel tube that slides INTO the existing barrel and is held in place by a special locknut on each end.
seen these for all models of colt percussion revolver, peace maker, and remington revolvers.