Baikal guns are being imported by Remington under the 'Spartan' name.
nowhere
Baikal isn't made buy Remington; it just has the name stamped on the side. It is made by baikal of russia
Online
Baikal parts are available thru EAA Corp. Below is a link to their parts store. http://www.eaacorp.com/manuals-parts-diagrams.html
You probably wont be able to. They would have to be specifically fitted to your receiver.
you can buy mossberg barrels in gun stores or from gun publications like, shotgun news, cheaper than dirt, midway supply, brownells, or go on-line to www.mossberg.com, and order them from the factory.
Will be hard to find, and expensive. Do an internet search for Corson's Barrels.
no you cannot. sabot slugs are made for rifled barrels. I would buy a rifled slug barrel.
Buy Shotgun. Buy Shotgun.
He can buy a shotgun, but, it is legally his until you turn 18
http://www.trulockchokes.com/cgi-bin/eShop/index.cgi
You could post in the classifieds on Gunbroker.com. When you find them (which you will, they are everywhere), they will have to be fitted to your frame by a very good gunsmith, and between the barrels and the fitting it will cost more than if you just bought a gun in good condition. Buy another good gun and you'll have yours for parts!