Your best bet is to post in the parts wanted or look in the parts for sale area. The cables are not sold separately from the software so you'll probably have to find someone who is no longer using their Turbolink sofware and buy the cable from them, or buy an older used version of Turbolink with the cable.
I think this is done to prevent piracy, since it isn't too difficult to copy the sofware, but very hard to copy the cable! The guys at Turbolink were always nice with me, but getting an extra cable was out of the question, understandably.
(Oh yeah, hope I can go to the next MAGNA meeting, last one looked like a good time).