At the very minimum for a pressurized system you need a cylinder, regulator, needle valve, bubble counter, tubing, and reactor. You can add a pH monitor and solenoid if you wish to automate the system.
For a cylinder check with your local welding shop, home brew beer shop, or the best place is a fire extinguisher shop. Try and find a place that will fill your cylinder while you wait and not just exchange it.
For the regulator, needle valve, bubble counter it's hard to beat the one at the AquaBotanic.com store. $99 for the whole unit. You can also get
CO2 proof tubing there.
For a reactor I build my own from
PVC pipe. There are some great threads on doing so over at the Aquabotanic.com forums. But to really make a
DIY reactor work you need to have a cannister filter. You can also buy a nice reactor at AB.