Needle valves

Hi Folks,

I am looking for a good source for a needle valve for a CO2 system.  I have a 16 
oz. CO2 canister that i would like to use for adding CO2 to the tank.  I need a 
way of regulating the flow and I figured that this would be the most COST 
EFFECTIVE (short of yeast) way of doing this.  Any other options, opinions, or 
comments would be appreciated.