I have yet to hear of anyone able to purchase the Viper, only advertisements for it. I am not sure that .22 shotshells are going to be at all effective out of that gun, or cost effective. There are no other air shotguns currently available (perhaps one of the Sharps from Florida importer might work, although last I hear they are unable to get the 9mm barrel's imported into the US).
In terms of an Air shotgun, as none are currently made, I would strongly suggest you try to find a Crosman1100, made back in the 60's. Are you familiar with those? IF not , I'll post a review of mine - but they are a HOOT to shoot, tons of fun - I use 9mm 56 gain pellets which are accurate (3 inch groups) out to about 35 (maybe 40 if pushing it) yards. Easily in excess of 20ftlbs at the muzzle. I also have a few of the orginal shells that I load up with either #6 or #12 shot - if I load them just right (takes a bunch of FUN experimenting), the shot stays in an 18inch pattern out to about 25 yards, certainly effective for small pest control.
I am currently seeking more orginal shotshells, fyi.
Let me know if you need more info on the 1100 - I have strong doubts that the Viper will ever be available, and if it is, if it will be anything other than a .22 pellet gun at additional cost (ie, useless in comparsion to the 1100 as a shotgun). I could be wrong of course