Firstly you need a CO2 cylinder, it should cost around $250-$300 depending on where you get it from. Then you need to regulate the CO2 coming from the cylinder with a regulator. Ideally your regulator would be dual gauge with a solenoid, anything less will be cheaper but a much harder to use. After that comes the check valve, bubble counter (optional) and diffuser. I prefer to get my gear overseas but others have had their cheap imported equipment fail.