Green gas is expensive, but a great alternative is a propane silicon mix.
Get a small tank of propane from Wal-Mart or something, some silicon oil (at least 90% silicon mix), and a propane adapter for airsoft.
Apply 2-3 drops of silicon oil to the top of the propane tank, attach the propane adapter and fill up your gun.
Green gas is literally propane pre-mixed with silicon oil then scented.