Speedometer Cable

Hey guys, I'm in need of a stock style speedometer for a 72 Demon. Ive been looking around at the local parts stores and they keep saying they can get the upper piece or the lower piece, but never the whole cable? Now I have zero experience with speedo cables so I'm lost as to what I need to make this work. I have the stock instrument cluster with the rectangular speedometer and a 727 trans hooked to a 360. Any help or guidance in the right direction to where to find everything I need is greatly appreciated! Thanks