They seem to be sold out almost everywhere, that's why I wound up selling my 500 that had a 24" rifled barrel. You should be able to order one directly from Mossberg for about the same price as everywhere else, but it might be backorder none the less. I'll probably buy a used 18" shotgun at some point down the road, since I don't have any immediate need for one.
Won't an 870 barrel fit? My 870 has a Mossberg slug barrel, and once you get over the light year gap in maching quality, fit and finish, it works just fine. I'd dare say it would work the other way, and you'd end up with a better barrel to boot. Should be easier to find too, 870 being the most successful commercial firearm ever produced and all. So I understand.