You may have to make your own. For some reason, Mossberg, who will sell you any sort of mall ninja shotgun with useless shiite such as pistol grips, heat shields, and ridiculous barrel attachments of all sorts, won't sell you a short barrel for your 20g or 410.
More likely, there just isn't a market for it.
And gunsmith can do it fairly cheaply so long as you are willing to live with toughup blue.