Plenty of plans out there. Shotgun bbls widely available, or you can use plumbing pipe. Check pawnshops for broken/cheap shotguns.
All the parts are out there for Remington 870/Mossberg 500 builds. Remington uses a steel receiver, but you could probably build it in aluminum. Mossberg Maverick 88 (500 w/o tang safety) would be the easiest, and the market might start making 80% Mossy 500 receivers if Remington goes under.