Are they not the same as the default one fitted to the ford mustang GT?
if so, I would think you will find them in stock at either Real Steel in Uxbridge or USAutomotive, both have websites.
Someone do please correct me if im wrong and they are different on the ZT fitment, but I would think like the alternator they are as per Ford Mustang applications to 2004?
see here:
http://www.realsteel.co.uk/section2b.pdf
part BYN3267 £96.89. not bad!