I would try the numrich gun parts corp.They can be found on the web.
On line auctions, gun shops, gun shows, pawn shops.
I have a complete 90% Crescent 12 ga 100,00 yours..
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There are some sources for Crescent shotgun parts, but it is very possible that parts will have to be repaired/made.
The wooden forearm is easily made, but if you need the metal part of the forend you are in for a tough time. We are one of the largest used parts dealers in the country and have not had a Crescent .410 forend iron in years. sales@countrygunsmith.net
Typical Crescent single-barrel shotgun. I have a good supply of parts. sales@countrygunsmith.net
The only Crescent shotgun that I know of that was nickeled was the 20 guage, 14 inch barreled Knickerbocker. A shotgun with a barrel length of 18" or less requires a Class C license from the federal Bureau of Alcohol, Tobacco & Firearms. Assuming you are correct and it is a 12 guage, it was originally blued and parts should be available from http://www.gunpartscorp.com.
Some Enders Royal Service shotguns are Crescent Model 60's marked with Shapleigh Hardware's trade name, but some were made by other manufacturers.
I have parts for that model. sales@countrygunsmith.net
Actually manufactured by Crescent Firearms. Parts might be found by checking some of the major used parts dealers.
American Arms was a trade name used by the H&D Folsom Company of New York on firearms they retailed circa 1900. It could have been manufactured by any of a dozen makers, but most likely by Crescent Firearms. Like any other 100-year-old tool or machine, parts are where you find them. You may be able to get parts that fit (or can be made to fit) from a local gunsmith or at a gunshow, but no one will have a catalog of parts for it.
Highly unlikely you will find any. Getting them made will probably cost more than the shotgun is worth.