It's not really a question of length. At the carb we use a barrel end that the cable passes through for the linkage and the cable gets captured by a screw to hold it in place. We just buy a VW cable assy, install it and then cut off the excess 12" or so. That way it's always the right length for your car (especially on a CMC!)
So you could buy one from Carey at Beck, or just get a VW version from CIP1, Bug City, Mid-America....etc. You'll probably end up cutting the end off anyway for the proper length. The Morse (Bowden) tube going from the frame to the fan shroud is the same length as a VW sedan so you can get one anywhere.