Too broad a question. Numrich Gun Parts carries a few parts for some models, look under both Merwin Hulbert and Hopkins & Allen (H&A made all of the Merwins so some parts should fit but you won't know until you try which is an expensive and frustrating solution.) www.oldwestgunsmith.com carries occasional original parts. Sometimes it's worthwhile to buy a non-working (different problem) model as a parts gun and those can be a few hundred dollars or less. The new Merwins will have some parts that fit a few of the original models, the large-frame revolvers only. www.merwinhulbertco.com
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Try e-gunparts.com
http://www.gunpartscorp.com/forum/
Try gun shops and gun shows.
You might try http://www.e-gunparts.com.
Numrich Arms Gun Parts Corp. West Hurley NY. is a good place to start
Numrich gun parts
Try e-gunparts.com
Gun shops, gun shows, want ads,
you know it beacause ,,,,find kenneth in BDC
Herter's imported revolvers made by Sauer & Sohn in West Germany. Try http://www.e-gunparts.com for parts.
Got it from a gun site message board. "U.S. Revolver Co. guns were manufactured by the Iver Johnson Arms & Cycle Works in Worcester, MA. from 1910 through 1930. Parts will not be easy to find, but try Numrich Parts Corp. at: www.e-gunparts.com"