If it's the same Portapro I'm looking at, it doesn't look like the cable is designed to be replaced by the end user. Given this, plus the fact that they're pretty inexpensive headphones to begin with, you're rather unlikely to find an exact replacement cable for sale.
Is the problem at the plug end, or at the headphone end? If it is at the plug end, just snip the old plug off and splice a new one (or one from another set of dead headphones that had problems at the headphone end instead) onto the cable. If the problem is at the headphone end, and you can get at the spots where the wires are soldered to the drivers without destroying the headphones, desolder the original cable, snip off the bad section, and solder the wires back onto the drivers.
The effort barely seems worth it for a $50 set of headphones, but if you're on a tight budget and already have a soldering iron and wire stripper tool capable of dealing with the fine wires in a typical headphone cord, it doesn't hurt to try.
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