try www.omps.it, they are the Italian main source of parts for the industry. Look up "aciarii" locks. There may be some semi fiished suitable back action locks.

You did not say if you seek hammer or hammerless locks. For back action hammer locks, if you are not too traditionally minded, the old Rossi coach gun had coil spring locks, with auto rebound system. They will need serious polishing but they should be easy to get in the USA.