Welcome to the Forum. parts for pre-1900 guns are getting tough to find. ebay often has various parts, but it requires you to search every few days until and if that part comes up for sale. gunpartscorp.com has lots of parts for old guns, as does Jack First, but as I recall they do not list large frame top-break S&Ws?
That part could be repaired by a competent welder as well, today's specialized equipment can perform exceptional weld repairs on very small parts in the right hands. A machinist can then shape the repair to fit your cylinder. This is the method I would use, since you could be looking for that part for years.
Also, don't forget to post a Wanted to Buy thread here:
http://smith-wessonforum.com/wanted-buy/