I'm not aware of any shotguns ever made with two barrels twisted together. If you are refering to "Damascus Twist" barrels, you need to learn a lot more about guns before buying one. Here is some information for you...please note the part about it not being safe to fire a Damascus barrel with modern powder. Picture an onion after a fire cracker went off inside it. http://www.hallowellco.com/damascus_twist_barrels.htm
Online
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.
Baikal guns are being imported by Remington under the 'Spartan' name.
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
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!
hasbro.com
GameStop man!
Yes, there are several sources on the intermet where you can find barrels. Japanese barrels are more recent, but even so, original Belgium barrels can be found at places like Gunbroker.com and Midwest supply.
Occasionally there will be a 10 Gauge shotgun for sale in your local sporting goods shop -if you don't see one, ask the clerk for a catalog- or possibly a pawn shop. If you are lucky, you may find an ol' double-barreled 10-Gauge, but some of them have "Damascus twist" barrels and can't handle modern loads.