Go to a compressed-gas and welding supply store, such as AirGas or Merriam Graves. Look on the corporate website for locations.

A small (24") CO2 tank is the correct size for a hot dog cart soda gun. And we all are caterers, right? I think I paid $170 for mine with the first fill. It has given me a lot of freedom in setting up my ventilation system; I know that CO2 is no longer a limiting factor in plant growth, even though the only grow space available to me after the last time I bailed out was a concrete cube with no freaking fresh air source... hahaha.

The regulator is available at a hydro shop. I got the Hydrofarm system, which comes with timer, regulator, and solenoid, and everything to deliver it with. It's really nice. Yay, CO2! Have fun with it, and don't forget that CO2 is heavier than air, so grow in the basement for safety, and always TETHER any type of compressed gas tank to a fixed object such as the wall or a sewer pipe.