Short Benelli barrels are notoriously hard to find.
You may have more luck finding a spare long barrel and buying a Tactical M2 that comes with a short barrel, and then selling your field gun.
If you check gunsamerica often enough there is a Benelli with 2 barrels about 5-6 times a year. That's how I got mine, but you sort of have to luck into them.
I don't know why they make it so hard to get spare barrels, and when you can find them they sure are pricey.