There are a lot of good sites for co2 tanks. One that I prefer is DicksSportingGoods.com they have a lot to choose from in many different sizes. They also have many other things besides co2 tanks.
Air tanks were invented to hold compressed air.
Compressed air is typically stored in a compressed air tank or reservoir. These tanks are usually made of steel or composite materials and are designed to withstand the pressure of the compressed air inside. The tanks can be found in various industrial applications, such as manufacturing plants, workshops, or even in vehicles equipped with compressed air systems.
Compressed air tanks were around long before paintball.
Compressed air can be found in scuba tanks, air compressors, aerosol spray cans, pneumatic tools, and air brakes in vehicles.
Firefighters typically get oxygen tanks from their fire department or fire station. These tanks are filled with compressed oxygen and are essential for providing breathable air to firefighters in hazardous or smoke-filled environments.
Most SCUBA divers dive with compressed air, which is normal atmospheric air compressed into a scuba tank.
It is a matter of buoyancy. There are large tanks that can be filled with sea water or with air. To submerge they pump the air into compressed tanks and the water fills it up sinking the sub. When they want to go back up, they blow the water out using the compressed air.
You can buy compressed air in a lot of places. Be it in stores or on the internet. You can buy compressed air quite cheap from many places, so look around and find what deals you like and see which you can get cheapest.
SCUBA tanks are metal (usually steel or aluminum) that are filled with compressed air. This is loosely related to a pneumatic system.
The same way they do today, they took saltwater into tanks while underwater, evaporated out the salt and electrolyzed the oxygen out of it. They also carried tanks of compressed air in the submarine.
"CI" on compressed air tanks stands for "capacity index." It refers to the maximum working pressure of the tank, measured in psi (pounds per square inch). It indicates the level of pressure the tank can safely contain without risking damage or failure.
The air trapped in the hull (this includes the buoyancy tanks) make it lighter than the water it displaces. The buoyancy tanks surround the main hull (where the people and machinery are), and can be flooded to allow the ship to sink. Refilling them with air (from compressed air tanks) allows it to return to the surface.