You can find a cheap gas airsoft gun online or at any large merchandising store such as Wall Mart or K Mart. Due to the rising popularity of simulation war games the demand for airsoft guns had increased tremendously in the past 10 years and many different types and sized are now readily available. As a word of advice, be certain the brand and the source are reputable before you buy one. Any flawed gas gun can be dangerous.
Gas airsoft guns fire BB's or pellets using a compressed gas such as CO2 or environmentally safe HFC. Their key advantage over traditional spring powered AS guns is the ability to fire ammunition semi automatically, without need of cocking the gun. They also can fire BB's at much higher speeds than the spring powered guns. Airsoft guns have been around for over 20 years, and began as high-end, near exact replicas of real firearms.
As popularity grew (especially in the paintball arena), there became a desire for the guns not due to their authenticity but because they can be used in battle. With cheap gas airsoft guns the startup costs is minimal-some guns go for as little as $20. It's important to note that the cheaper the gun, the less powerful and less accurate over longer distances the gun will be. Cheap guns fire BB's at around 120 feet/second, a far cry from the more expensive guns that fire at speeds exceeding 500 feet/second.