Do you have any Cabela's near your location?
They have a pretty decent Mossberg 500 Home defense/Field Combo. It comes with a smooth-bore 18.5" heavy breacher-barrel (Cabela's website says 20", but it is 18.5") and a 28" ribbed field barrel w/ 3 different screw-in chokes (with choke tool). I picked it up, on sale, in early December for $349. (go to Cabela's site and look under "Pump Shotguns" and then under "Mossberg Tactical Shotguns")
Here is mine (added a tri-rail and light)
Only thing I didn't like about it was, and something you should keep in mind when looking at Mossberg 500's.... a lot of their newer models use the Maverick 88 style pinned forend (if you look above, you can see the pin going through the forend) . Although perfectly functional, it poses a problem if you want to change the forend, as the pinned forend is one piece with the action bars. So, keep that in mind when shopping for a 500 and ask, or look for yourself, if in person, if it has a pinned forend or an actual action-tube/slide-bar configuration (if that bothers you).
I ended up upgrading the stock and forend to a Hogue over-molded set and it required me to purchase a separate action slide tube assembly (Promag #PM194 for $40). Added money, but when considering I got the nice breacher barrel and a nice ribbed barrel with chokes for not much money, I'm happy with it.
here it is as it stands now . If you look close you can see the nut in front of the forend that attaches to the metal action tube.
Mossberg also has several other combos that come in various configurations .. but some of them have a fixed-choke long barrel. So, I like the Cabela's version the best (and the breacher barrel is pretty bad-$%# :P )