I just ran accross this on ebay, I don't know if the whole assembly is the proper one for our cars but the lights look like they are in good condition.
http://cgi.ebay.com/ebaymotors/67-Shelby-GT-500-Tailight-Assy-68-California-Special_W0QQcmdZViewItemQQcategoryZ38656QQitemZ8026982055QQrdZ1
http://cgi.ebay.com/ebaymotors/67-Shelby-GT-500-Tailight-Assy-68-California-Special_W0QQcmdZViewItemQQcategoryZ38656QQitemZ8026982055QQrdZ1