That's gonna be a tough one. Maybe David Chicoine (
www.oldwestgunsmith.com) will have one or know of some suggestions. I've never had one of those apart, so I don't know if an eary I frame (Model of 1903) hammer would work or not. A .38 Perfected (top break with side thumb latch) is very close, too, but I don't know.
How is it broken? Could it be welded and rebuilt? That might be your best option. (Again, talk to David. He could probably do that work.)
Good luck!
Chris