Sell it. People are always looking to "tac out" shotguns. List it for sale with "tactical upgrade, tactical conversion barrel, riot shotgun, street sweeper, sawed off shotgun, home defense" tags. In the item description be VERY clear and honest in what it is, and what the condition is, but those tags should attract the searches of the kind of person that will be shopping for that kind of barrel. Odds are you can probably get enough out of it to cover the cost of your new barrel.
OP, the standard mossberg 18.5" barrels are back in stock for $120 before tax/ship:
https://www.mossberg.com/store/500-12-gauge-security-barrel-18-5-blued-6-shot-90015.html
compatible with your gun if it has the 6 shot magazine tube