CO2 cylinder refills are most commonly available from sporting goods stores and from directly on the paintball fields where they are used. Several companies can handle refilling these cylinders from both on and off the field.
Chain sporting goods stores such as Dick's Sporting Goods or Sports Authority can fill the canisters for a charge. Most paintball fields also have on-site refill stations. The charge for refilling in either case is based on the size of the tank, but this cost only amounts to a few dollars regardless of size or location.
Some locations sell kits for refilling canisters manually, but the customer must also have a larger compressed CO2 tank he can use to fill the smaller canisters. This allows a player to fill his own CO2 canisters, but unless he plays the game very frequently anyway, this convenience is offset by the high costs of these kits and the large tanks.
If a player cannot seem to find a place to fill his CO2 canisters, there is an alternative type of business to search for that can provide the service. Local companies that provide fire extinguishers and fire safety related equipment also usually have the means to provide CO2 refills.