Try Brownells or Numrich Gun Parts online.
you upgrade the gun and then you have "upgraded ammo." it's simply ammo for the upgraded gun, you can't buy it separate. you just have it after you upgrade the gun
Gun shops, gun shows
14 gauge has been made.
Personally I'd say the gun itself decides that haha, but generally speaking the 16-gauge is a good multi-purpose shotgun size, it (with the proper ammo i.e. slugs) can take down a deer no hassle.
Brazil 12 gauge single shot gun
The 32 gauge shotgun (NOT .32 caliber) is fairly rare in the US. They are more common in South America as a bird gun.