You have to check from time to time. Try Sinister Sports Paintball in Roseville. This store is small, but it mainly deals with paintball sport related items. That's where I got my Black CP used On and Off Valve for dirt cheap.
Bigboytoys in Elk Grove carries Half Paintball related sports and the rest are Nitro/Electric RC Cars.
You can always call Sinister Sports and ask if they got any used ON and Off Valves. I only mentioned the Paintball store in Elk Grove because I"m closer to there and I only go there to refill my Co2.
Was that the cheapest price for a new On and Off Valve?