Mossberg will sell you a Model 500 barrel for $74, 18.5" plain bead-sighted, cylinder choke. You can order directly from Mossberg:
www.Mossberg.com
or 1-800-363-3555
Part number 90015
I bought that same barrel from my stocking Mossberg dealer for $69, you may be able to find it for less.
My personal preference is NOT to cut down barrels. I tried that once, many years back, and I ended up having to replace the barrel. A bit of mud, unnoticed near the muzzle, gave it that "Blunderbuss" look. [Don't ask how I know, I just know]. I never could seem to get the cut straight, so I finally gave up and bought a new one.
Who knows, at some future point in time, that Mossy may be used as a regular field hunting weapon...
While I like the short (18.5") barrel on my Maverick Mdl 88 for a Homeland Security Application, it is somewhat impractical in the Waterfowl Blinds.