When I got my C20 it had the factory air stuff on the body but nothing on the engine. I bought all the stuff from here:
http://www.nostalgicairparts.com/
I believe I spent about $300, which included brackets, a new compressor, hoses, and the valve replacement thingie.
I work quite well, blowing 36 degrees. My only complaint is the factory center vent in the dash. It only blows straight ahead, which is annoying. I'm considering replacing it with 2 vents so I can aim one at myself and one at the passenger.