Here is a place in the US:
http://www.precisiontandems.com/catframepart.htm

Here is a place in the UK:
http://www.tandems.co.uk/products/ac...em-gear-cable/

If I was a cheap bastard, and I am, I suppose I would try to tightly twist the cable and use a nut, bolt and a couple washers to hold it together. Maybe even put a groove in the washers with a hacksaw or Dremel to act as a cable guide of sorts.