CO2 cylinder

go to the local gas supplier--he will charge you a one-time deposit. but, then all you do is trade cylinders and pay for the gas.

There is a site--aquatic??? that sells them.

Or, go to a local home brewery supply shop--they should sell them. you will pay a premium there.

edit. it is premium aquatics--http://www.premiumaquatics.com/ .

still cheaper to just pay the one time deposit. that way you do not pay to ship a heavy, empty cannister.

I bought a cylinder, but if I were to do it again I would do an exchange with a local shop. Not all places will fill your own tank on a walkin basis. Plus you do no have to worrie about the 5 year inspection.