OP, there is an *almost* complete Mossberg 500b on Gunbroker right now: Mossberg 500B .16ga Complete Receiver - Pump Action Shotguns at GunBroker.com : 875091889

Starting bid $100, no bids so far, doubt there is going to be much competition. It is missing it's barrel, and that won't be easy for you to find. But if you do buy this one, you will have the regulated part, and if you find a barrel on eBay or Shotgunworld.com or someplance, you won't have to go through an FFL.

Good luck!