I actually think he's after the original white one that came with the Mac Pro, there was another thread about this recently. I believe your best chance of success for such a thing is once again going to be Amazon.
The white cables you find on Amazon are however of a slightly different design. The original cable had a square shaped plastic end to fill the power socket completely.
If you're really desperate for the perfect cable then you're probably going to have to look on eBay for that.