you are looking for a very hard to find item. Keep looking at all the shotgun pubs you can find, join the various associations(collector and shooter) and check the newsletters. Also check wth high end gunsmiths that specialize in trap/skeet/sporing clay guns, they might have a bulged or burst barrel that can be shortened.
The one you want is marked cyl bore, can be 2o or 21", ad ill be 2.75" chamber. Hope you find it, morelikely you will have to make do with shortening a longer barrel.