Brian,
If you purchase a new RealFlight G2-Interlink, it comes with everything you need, including 9C-compatible interface. The downside--it's pretty expensive.
An alternative: try to find used pre-Interlink version of RealFlight G2 (i.e., G2 originally sold before April 2002 with either parallel port interface or dummy game port controller) and buy software only. Just make sure that the seller un-register the software from RealFlight database (I've contacted RealFlight support and they told me that, if the original owner relinquish his/her registration, you can register the software yourself and enjoy full tech support).
In this case, you may order a more affordable TX-to-PC interface from any of interface vendors. Just make sure that their interface is compatible with RealFlight. I've had good experience with interfaces from
www.milehighwings.com .
Hope this helps,
Boris