Try e-gunparts.com There's a whole list of places on the Remington Society of America here: http://remingtonsociety.com/rsa/service
Check this page on the Remington Society of America for a list of places: http://remingtonsociety.com/rsa/service
NUMRICH GUN PARTS SELLS THEM ON THEIR WEB SITE
Check this list on the Remington Society of America: http://remingtonsociety.com/rsa/service
The Remington Society of America has a services/parts link too but e-gunparts.com is a good place to start.
Not marked, Model 9
1920
Remington Model 12 barrels are threaded to the action. Barrels can be replaced with the proper equipment.
Model R-12
stocks are the same for any model 12 or 121, try e-gunparts.com or watch on gunbroker.com. There are people making them new as well for example http://macongunstocks.net/remington_model_12_or_121_2_piece_riflestock_96.html http://www.gunstocksinc.com/web_pages/Plate-pics/Remington-Stocks/Remington-Rifle-stocks.htm You can also try e-bay. Keyword Remington Model 12 and see what you might find.
Try Remington's web site. Go to remington.com and order one, they are free for most guns.
Numrich gun parts. Numrich has a nice exploded view of the SPT 58 & parts list.
No such model is listed in the Blue Book or Standard Catalog.