There are plenty of aftermarket companies, as well as Ford making some mighty fine plug wire separaters. Which ones look good depends on the look you want under the hood. Chrome, color matched etc. Have you checked out either Brothers, Jegs performance or Edelbrocks website. They both have a good selection. Here's the links:
http://www.brothersperformance.com/ http://www.edelbrock.com/ http://www.jegs.com/
Check here for some other great links that might be of help
http://www.allfordmustangs.com/links/
Hope this is of some help
wire separators 
