To find cheap CO2 tanks, look for sales at paintball stores, paintball fields and sporting goods stores. Check both in-stores and online, as sales may only be offered at one or the other. While a used CO2 tank may be cheaper, it's safer to buy new tanks. It's more cost effective in the long run to buy the largest CO2 tank possible, as refills are usually less expensive per ounce when refilling larger tanks.
CO2 tanks are widely available and are the most cost effective method of powering a paintball gun, so it's not difficult to find inexpensive CO2 tanks at many locations that sell paintball equipment. CO2 tanks have internal o-rings, and the tank isn't safe to use if these are damaged. For this reason, new tanks are safer than used tanks, as there's no way of knowing if the used tank's internal o-ring is intact.
CO2 tanks start at 4 ounces, which can provide about 150 to 200 shots. Larger tanks, such as 20-ounce tanks, can provide well over 500 shots. However, these are only estimates, as a variety of outside factors affect the amount of shots a tank provides.
Stores that sell CO2 tanks are usually also able to refill them. Tanks must be completely emptied and then refilled, so it's best to use as much of the tank as possible before having it refilled.