Gun shop, gun show, pawn shop, want ad, for sale ad, garage sale, estate sale, on line auction.
You will have to contact Rossi or find one of their catalogs
On line auctions.
Try e-gunparts.com and Marlin
Should be on the barrel near the receiver.
You really need to find a factory trained gunsmith.
Try e-gunparts.com
You will have to contact the Henry factory to find out.
You can order one through Cabela's or you can contact "Wild West Guns" in Alaska.
You must have a Western Field Model 72C lever action rifle, which is a Mossberg 472C. Try gunpartscorp.com or eBay for an owner's manual. A repair manual is going to be tough as the gun is not well known.
Flayderman's Guide to Antique American Firearms
Try gun shops, gun shows, want ads, e-gunparts.com
The Stevens 311 is a double barrel shotgun.