I've seen them before, but the problem with the Mossbergs is that you don't just add a mag. tube extension like on the Remingtons, but rather a whole new longer mag. tube. Then you need a different special barrel to accomodate the longer mag. tube where the barrel lug goes over the end of the mag. tube at the point where the mag. tube cap screws onto it. Not many places sell that sort of thing though. That's why the Remingtons are so popular, Mossberg sort'a screwed themselves on that design.
With the +2 extension on a 18 1/2" barreled Remington I get 6+1 shot, in my standard Mossberg with an 18 1/2" barrel installed I get 5+1. The one shot difference really isn't worth the extra cost or trouble to get an extension, so I just live with it.
You're more likely to find a 18 1/2" barrel for the Mossberg in the standard mag. tube length configuration than in the extended configuration. The 20 ga. part puts a whole extra twist on finding one though. As I'm sure you've already found out, 12 ga. barrels are plentiful, 20 ga. barels are tough to find. That's somewhat weird too, because IMO the 20 ga. makes an ideal HD shotgun.