Amazon.com : AGGXPF Bright red/Green Laser Bore Sight Kit for .17to12GA Caliber, Laser Boresighter Kit with 12 Multiple Caliber Adapters, Laser Bore Sighter for Hunting Zeroing Your Scopes Optics (Green Laser) :
Chiappa X-caliber 12ga Adapter - Set 8 Inserts With Case - Barry Paul Manno - FFL
Eagle Gun Range -Macon GA - X-Caliber Ammo Adapter Set for Shotguns. It's an 8-piece set of drop-in adapters that measure seven inches long each, and it comes with a carrying case.
The Utility of 12-Gauge Caliber Adapters, The Good, Bad, & Ugly -The Firearm Blog