So my question is, when I go to Radio Shack ("You've got questions, we've got ridiculously expensive cables") tomorrow, what do I buy?

ROFL, good one.

If it were me, I would buy the necessary parts to assemble my own cable with the necessary male and/or female connectors. This would allow me to use trial-and-error with the TX and RX pins until it worked.

Remember that you only need to connect three pins total: Pin 5, which is ground, and pins 2 and 3 which are RX and TX (and are the ones you swap with trial and error until it works right).

Only way to be sure.