If the big cable from the relay to the starter is all you need, its just a standard 60" 4 gauge cable.. Auto Zone has them, you can look it up by vehicle on their webpage, or they can look it up for you at the store. The Auto Zone part number for the cable for an 1986 5.0 is DW460B
I just have to ask.... What is it doing/not doing?
Just to be sure we're on the same page here...
The only wire going to the starter is more like a really long battery cable.. big wire.
One end connects to the starter relay (solenoid) on the drivers side core support, and goes across by the oil pan, then back to the starter.
The starter relay has 3 wires on it.. the + battery cable, the long cable going to the starter,and a signal wire from the ignition switch. The signal wire from the ignition switch powers the relay, which in turn connects the positive battery cable side of the relay to the starter cable side.
Make sure the car is in NEUTRAL or PARK... then take a screwdriver and connect the 2 large posts on the starter relay, and if the starter turns, there is nothing wrong with the cable going to the starter or the starter itself, but it is the relay or the ignition switch itself.
To rule out the starter relay, you can take a screwdriver and touch the post on the solenoid with the + battery cable connected to it to the small post, which is the signal wire from the ignition switch. If the starter turns, that signal wire is not giving power and the starter and relay are good. Its either the ignition switch itself, the neutral safety switch, a clutch safety switch, or a shorted wire somewhere in between.