You may find a used 45 ACP cylinder at gunshows or on Ebay. That is very risky. Some times they fit, sometimes they don't. You best bet is to contact Ruger and see if they will fit one for you. Last time I checked, they wanted $80 for the cylinder and labor to fit it. Of course you will spend that much more on shipping.
This is really a question for Iowegan...he can give you the definitive answer. I don't think Ruger will sell him one, but he should be able to find one at a gun show. It can be done. I did just the opposite. In the 80's I found a really nice 7.5" Blackhawk convertible really cheap. The only problem was that the .45 Colt cylinder was missing. I picked one up at the Tulsa Gun Show later that year for $25.00 and fitted it to the gun with a flat file and a lot of careful filing and measuring. It worked out great but it was probably more luck than skill. A gunsmith I am not.
Thanks for the responses-if we do it-we'll do it right-I ain't no shade-tree gunsmith and almost kinda scares the crap outa me!! I doubt he'll spend the $$ but that's the only option I'll report to him...what he don't know won't ever hurt him
Well, IMHO, the $325 is a good price but no bargain unless it's NIB. Why not keep looking for a convertable BH and get it right from the factory. You'll save the cost of fitment, and either come out with two guns or trade/sell the 7 1/2" bbl one and get your money back. Might even make a couple bucks profit. I guarentee you can get a good NIB 45c/45acp for less than the $450 or more you'll end up spending by the time you get the spare fitted to his current gun. Gun $325 + 45acp cylinder $80 + Shipping $45 (average)= $450