I've bought Duracoat at Midway USA for years, and I've also bought some directly from Lauer, the manufacturer (they're located in Chippewa Falls WI) when Midway didnt have a color i just had to have.
A word of caution....
After you do the Mossberg, you'll start buying lots of "junkers", just so you can fix them up, and "paint" them. It's addictive.. i've got a safe full of them.
Fwiw...I use an airbrush i bought way back in the 1980s to do lettering and custom paint on commercial trucks, and I did a few snowmobiles with it.
Also, don't waste money on their cleaner/degreaser. Non chlorinated brake cleaner from Walmart or Menards works just as good, if not better. Whatever you use to prep it, do it twice just to be sure. Proper cleaning and prep is the key to it adhering and staying on.