gunbroker.com
www.urban-armory.com
I would first look at the numrich gun parts corp.They can be found on the web.
Made by Savage/Stevens and is basically the same as the Savage Springfield Model 18.
E-gunparts.com
Hello: Your best bet is to watch the gun auction websites for parts or non-working parts rifles that have the parts you need. Be aware that Savage made several variations of the Model 3. And parts between the variations don't always interchange. Savage also market Model 3 variants under the Stevens and Springfield maker name (both owned by Savage). It took me 2 years to find bolt parts for my early Model 3.
www.savageshooters.com
You can find a magazine for Savage/Springfield Mod 18C-410 Ga at e-gunparts.com, then NUMRICH iten number229930A and they cost $37.10
One of a long list of variations of the basic Savage/Stevens semiauto made for fifty some-odd years. Depending on what parts you need, we probably have them. sales@countrygunsmith.net
Check www.e-gunparts.com
gunpartscorp.com seems to have everything you have to check your model number but most of the parts seem to there
Egunparts
That model is obsolete, but we have a good selection of parts. sales@countrygunsmith.net