You could try Barbara Minor and Chris Hentz. They hand make preciousmetal cables. You can contact them and see their work heRe:http://www.chriscables.com
Try Herco in San Francisco. They have 18K cable in a few differentdiameters.
Have you considered, buying a tungsten drawplate and drawing thewire yourself? In the long run, the cost evens itself out. of courseyou do need a rolling mill as well