The ST170 has Xeon lights as standard, guess you have looked on Ebay etc? You may be able to replace with a standard headlight but really you would have to do the pair and by the time you have done this it's probably cheaper and easier just to buy a replacement Xeon, there is a trader recently joined on here "Carparts Express" that promise to be cheaper than Ford and so it may be worth giving them a try.

Martin