Buy any used 11-87 barrel and cut it down to length...
You can do it yourself if you're careful with a fine blade on the hacksaw, and fine file the edges very neatly with a very fine fine.
Take a look at http://www.remingtonle.com/shotguns/1187.htm . Whacking off a longer gas gun barrel as short as 18" is going to require adjustments to the gas ports, and that is not likely a DIY deal...