Try e-gunparts.com, gun show, gun shops, want ads.
$50 to $100 depending on condition.
This model of shotgun is identicle to a mossberg 395. You can still purchase clips for this model online or at gunshows. There are 2 round and 5 round clips availible. Just search for the mossberg 395 clips.
75-225 USD
There is a ammo clip for a J C Higgins shotgun model 58324 16 gauge. You can buy it as a replacement part.
NUMRICH.com
$50-$90
We carry new replacement magazines for that shotgun.
Your Montgomery Ward's shotgun was made for them by Mossberg, and is likely a Mossberg model 85 or 85b. Gunpartscorp has 20 guage magazines on their website (under Mossberg, Model 85b) for about $32 plus shipping.
50-160
Mossberg or Savage/Stevens? Any particular model? sales@countrygunsmith.net
http://e-gunparts.com/ has themNo they do not.
about $100 if its complete, clip and all. not many people are looking for a bolt action 20g. there is one on www.gunbroker.com