Egunparts and the original maker.
Optics Planet and Leapers are two of the places that have parts or can order in parts for this type of gun. Parts can also be purchased on Amazon and sometimes on Ebay.
You can order parts at www.wisnersinc.com
Go to a gun shop
Its a used gun which has been either cleaned or parts replaced in order to make it fit for resale.
Places to try would include Marlin, gun shops, gun shows, e-gunparts.com
Find a good gun shop or gun smith.
Gun Parts Corporation is the main supplier of Numrich gun parts. On eBay, one can find Numrich gun parts. On Amazon, one can find a few Numrich gun parts.
Depends on which parts, for which pistol. A good start would be your local gun shop. You may be able to order parts from the manufacturer, or gunpartscorp.com. Some parts can be installed by the maker, but are not sold to others because of the need for fitting.
Same as a Marlin 336. Order parts from Numrich gun parts, Hurley, NY
Try on line auctions, gun shops, gun shows, pawn shops.
One can get spare parts for a Nikko shotgun at some local gun retailers or through gun manufacturers. Gun parts can also be found through gun classifieds and gun auctions.
A Sears Model 300 is a Winchester odel 1400. Look for parts at: Numrich gunpartscorp.com Jack First jackfirstgun.com Bob's Gun Shop gun-parts.com