Or pick yourself up a Oxygen or Acetylene tank at a local Welding Supply company/shop if you want a Tall/Skinny tank for portable (and armored) use. They're rated for several thousand pounds of pressure. The only downside is they lack a drain on the bottom, and trust me you want to drain the water out of the tank regularly. Although, to be honest, you could always flip the tank upside down and drain the water out through the valve on the top and then refill it with air again.