I own both the 28" VR blue/walnut and the high capacity FP.
The tubes and where the barrel "mounts" on the tube are diffrent.
Brownells offers magazine tubes, but they are expensive. Used barrels are few and even more expensive! There is a guy who runs shotgun barrel ads in Gunlist and Shotgun News ("Corzine's" I think).
IMHO the Beretta inertia action shotguns are amoung the finest and fastest shotguns available. There was a professional shooter on the TV using a 1201FP for trick/excibition shooting, man was he fast!
Sadly for the price of a tube and even a used barrel you could proabably buy another shotgun. If you find a deal on barrels, my 28" barrel isn't fitted for tubes, and I wouldn't mind having one that was.
On the subject of magazine tubes: Of three 1201FP's I've owned, this one had the magazine tube roll crimped ~3-1/2" from the end, which decreased the capacity....strange.