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. Local companies that provide refills for products such as fire extinguishers and fire safety equipment or companies that deal in other compressed gases usually also provide services for CO2 refills, and they can also be found in the local yellow pages. The charge for refilling in any 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 without having to rely on sellers unless his supply has run dry. But unless he plays the game very frequently anyway, this convenience is offset by the high costs of these kits and the large tanks.