[Arran]

Sometime the old cloth covered cords are still usable, where the jacket is in good condition and the wire itself is nice and flexible, those you can attempt a repair on or cut the bad portion out at either end. In terms of the old rubber versions of a two conductor zip cords, once they are gone they are pretty much useless for anything but an antenna wire. I've had rubber cords that were so bad that someone had coiled them up in the back of a radio and the coil had fused itself together and become petrified.
There is an outfit called Sundial Wire that sells a lot of reproduction cloth cord for antique lamps, fans, and other appliances. For replacing the old rubber ones try the hardware store or home center, but maybe get the 16 gauge cord rather then the 18 since it's often closer to the original rubber cords in size. For texture you can try appliance cord, it seems to be more like rubber then plastic, however it usually comes in one colour, black.
Regards
Arran