Like I said, call Mossberg. Per an old TFL post you might want Mossberg part #90823.
I was looking for an 18.5" 590 barrel to go on my 835 when I got lucky and found an actual 18.5" 835 barrel with a 3-1/2" chamber on GunBroker for about $80.00. The Mossberg part number for the 18.5" 835 barrel is #90823 and you might be able to order it directly from them.
You may also want to look at http://www.mossbergowners.com/viewtopic.php?f=15&t=1170