It's been a while, so I'm not sure if all of these places are the cheapest anymore, but these are some good places to get stuff:
Milwaukee regulator/solenoid/needle valve/ bubble counter from
www.water-testers.com
CO2 cylinder from
www.beveragefactory.com or look around locally. Usually, local is the best way to get a cylinder. Usually cheaper or at least comes filled with co2.
Other than that you only need a way to inject it. You can use a ceramic disc diffuser, cheap on ebay and you'll need some airline tubing.
Finally, without a drop checker, you're flying blind. Search "drop checker" on ebay, you can get one for about $13/shipped. You can probably get the diffuser from the same seller and save on shipping.
It won't be the top of the line equipment, but it'll give you a pressurized system for under $170.