I don't know how critical the end-to-end resistance value is,
but I suggest that you get a 100K stereo pot from Radio Shack
(#271-1732) and give it a try.

Though a 100K pot will almost certainly work electrically, its source
impedance is twice as a high as a 50K pot. This, combined with the stray
capacitance of the wiring following the pot, might audibly roll off the top
end.