You might check with KAR (Columbus OH) or Glazier mustang barn (east PA) to see if they have a parts car. I would think that you could find a roof from a non vinyl top car that could do the trick. There is also a Pittsburgh mustang parts retailer but I do not have a web page or phone number.
http://www.karmustang.com/Default.htm http://www.mustangbarn.com/home.htm