It's typically called an armory.That is the Arms Room or Armory.
Gun shop or sporting good store
Sacristy
As many as they can afford and have room to store properly. You can only shoot one at a time.
pantry or kitchen
A pantry
You can get water guns at your local store. Such as walmart or target.
A gun store
They purchase them at a gun store.
The guns used in war to shoot down planes are called antiaircraft guns.
Machine guns.
The Dollar Store, Walmart, Walgreens, and Target usually have them. :)