Radio Shack used to sell such a thing, but I very much doubt they still do.
I expect you could rig something very similar to the SEX amp arrangement. You will basically need a stereo headphone jack (TRS), 120 Ohm resistors to put in series with the T and R connections, and whatever makes sense in terms of leads to hook up to your amp. A little box to put the jack and resistors into would be good, of course. Radio Shack's used to come in a little metal cylinder with about 6" of speaker wire coming out the end to connect to the left and right outputs of the amp. Any little project box big enough to hold the jack and resistors would do, I imagine.
