The first place to check would be at a local gunshop. If that fails, try Numrich Gunparts
The magazine holds two shells.
Online
If you can find one, yes.
40-100 USD
Your shotgun was made for Sears by Savage, and is their Model 18 Shotgun. Magazines are available from gunpartscorp through their website. The part number is122990B, and the cost is $32.75 plus shipping
http://e-gunparts.com/ has themNo they do not.
2
You can purchase parts for a Ranger model 30 16 gauge shotgun at these locations. Gunbroker, Topgunsupply, Budsgunshop, Gunpartshop and Midwayusa.
Try http://www.canadiangunnutz.com/
Where can I buy a magazine for a western field M149 bolt action
I payed $400 for mine so I would use that as a starting point.
Pretty much a custom proposition. Do a google search on custom shotgun makers and start making phone calls.