I've seen the XT's at Wal-Mart, so give them a try. As far as the 981T, you will need to check pawn shops and gunshows. The XT's replaced the 900 series I believe. Haven't looked at Marlins site to be absolutely sure. Haven't seen a 981T new on the shelf for about 8 months. I have seen a few 982T, 22 mag, just a few months ago.
Now that I think about it, its been more than a few.