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Ruger may do a spare cylinder for you IF that model is also offered as a convertable. But, you have to ship them the gun, and it can be a bit pricy.
Many folks have found that they can buy an extra cylinder for less, and have it fitted. Or many of us can fit one ourselves. Many will also drop in w/o any fitting. First, it must be the same OAL, or a few thousands over. Next, you have to check the timing. If you don't know how, get a gunsmith to do so.
Cheapest option is to buy a convertable already built.