20oz? Yo mean like a paintball tank? If so you're going to need to find an adapter in order to get a regulator to connect to it. Do a search on Paintball and CO2 and you should find info on the stuff you'd need.
So yo do have the regulator already? You can get just the solenoid from places like Aquariumplants.com. I believe it'll have female 1/8" NPT threads so you'll need the appropriate nipples to connect it to your regulator and bubble counter/needle valve.