Might not be too many members with experience that fits.
From Tower's tech info:
this package also includes a RealFlight NexSTAR Windows PC disc with transmitter interface to use with the included Futaba radio
If you got that response from Great Planes/Real Flight, then I'd say they aren't selling any more of whatever cable they included. RCU has a want ads section. Maybe someone with there could help.
Also, you mention that you have an old Futaba RX you could use. The Nexstar deal included a transmitter and cable to use with that specially written software. GP/RF's piracy protection concept requires that the user start with an original purchase package. If you contact them directly, they should be the best ones who might be able to help you.