Your gun is going to require air to shoot, and CO2 is the least expensive and most readily available source out there. CO2 is available at 1000's of stores nationwide. CO2 is available at sporting goods stores, paintball supply stores, hardware stores, bulk air/gas retailers, welding supply shops, etc.
Tank Features:

• High quality aluminum CO2 Tank with industry standard pin valve, just screw it in and you are ready to go
• Approximately 375 shots per fill, 10.5" length and 3" width
• 1.75 lbs

NOTE: This gun will also operate off of HPA (High Pressure Air) or Nitrogen. CO2 Tanks are not designed to be filled with HPA or Nitrogen. HPA Tanks / Nitrogen Tanks are available at most sporting goods stores and paintball supply shops.