The cable is the same as the one used for years on Radio Shack computers - from the TRS-80 model I to the color computer. So you might find one on ebay or someplace like that, although they haven't been a current product (pun intended) for years.
If you feel like making your own, there are instructions here: http://www.8bit-micro.com/data-cable.htm
If you feel like making your own, there are instructions here: http://www.8bit-micro.com/data-cable.htm