Try Gallenson's in downtown SLC for the ammo. They carry military surplus stuff. Otherwise you can wait for the next Gun Show (September?) and find it there. Jus FYI, buying 1000 for all those calibers is going to cost you a pretty penny (~$600).
As per the tactical shotgun, if cost were a factor (I'm assuming it is) I would get one of the Chinese shotguns from FBMG. For the price, you can't beat the performance. There's also the Saiga-12 which looks like an AK and has a 10 round mag - can't get that in another shotty! If you are looking to up the price/quality a bit, you can get a Mossberg 590 or Remington 870. Call FBMG and ask about their shotties though, as I recall they had a pretty sweet deal on their Chinese tactical guns.