You need a "3.5mm mono mini-plug" cable (male to male) -- in other words, a cable with small headphone plugs on each end (the mono kind, not stereo). It needs plugs that look like this on each end:
(note that you only want one dark band at the end, not two -- two would indicate a stereo plug)
I couldn't find a cable on Radio Shack's site or PartsExpress's site (still looking), but they are available at Radio Shack. I actually already had one lying around from who-knows-what...
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Gonk