🚀 go-pugleaf

Actually, Radio Shack DOES sell a cord that may do the trick for you.
From the 1990 catalog p.29:
Dubbing Cords
Attenuating with 1/8" plugs. 6.5 ft.  #42-2152 $3.39

Otherwise, it's pretty easy to make an audio attenuator.  Perhaps the easiest
thing is to stick a volume pot (10 kohm - 100 kohm, somewhere in about that
range ought to do, taper doesn't really matter) in a metal box with a couple
of appropriate jacks (or shielded  cables with plugs, if you wish), in the
following configuration:


signal in>----------|
                    /
             pot    \
                    /
                    \<-----------------<signal out
                    /
                    \
shield (ground)     /
in >----------------|-------------------> shield out

and then adjust the pot until you get no distortion at the input of your 
mixer.

You could also use a pair of fixed resistors instead of the pot, say, 20k
to the signal in and 1k to ground should give you enough attenuation.  But
the pot is better, because you can adjust it to the right amount of atten.
-jim