Hydrogen is the lightest gas in nature, with an atomic number of 1. Hydrogen is the gas that provides the most lift per volume. In addition, hydrogen's abundance in nature makes it extremely inexpensive and easy to obtain. However, hydrogen is a reactive gas that is highly flammable and possibly explosive, especially in high-oxygen environments. Although hydrogen may seem like the best gas to use in a balloon, you may wish to consider the risks and hazards of using it before proceeding. If you decide to use hydrogen in you balloon, special precautions must be taken to prevent the spontaneous combustion. Be sure to ground your hydrogen tanks to prevent stray sparks. Also, make sure that oxygen is not allowed to mix with the hydrogen, as a particularly violent fireball may ignite. When in doubt, remember: anything that may cause a spark, a flame, or a shock has the potential to ignite the hydrogen.
We recommend helium for usage in high-altitude ballooning projects. Although it only provides half the lift of hydrogen at twice the price, helium is an inert gas and does not ignite or react with air. Helium is also preferable in some situations where balloonists want their balloon to rise more slowly. The only hazard of helium is inhalation, and this hazard is virtually non-present at all launch sites.
Gases for ballooning can be obtained over the Internet from many suppliers that cater to both scientists and amateur ballooners. Praxair is a common supplier for industrial and scientific purposes. However, this supplier is not accessible for amateur ballooners. Check your local business listings for balloon suppliers or helium producers.