Years ago I drove to every parts store in my area with no luck. I took my OEM cable and went behind the counter and matched the ends up to two different cables, then cut and spliced the two to make one I could use. $12 for one cable, $8 for the other.
While on the subject, you can add/subtract 15% of your speed-o-meter by buying a reducer from GM for a 1987 MontyCarlo SS. It goes between the cables by the firewall. To make speedo read 15% lower, take apart the gizmo and flip the gears around inside. (like for 3.73 or 4.10 gears)
If you stay with stock rear gears and add 28" tall tires, you install reducer as is, and speedo will read very close to perfect.