Specialty Mustang Designers-Outfitters:
Steeda, Saleen, Rousch, Ford-SVT/Mach/FR500 , Sean Hyland Motorsports, Shelby..
Individual Mod Companies:
MSD, Crane, Cervini's, DallasMustang, Jegs, Summit, Pro-M, FRPP, Hypertech, Jet-Chips Inc., ATI, Vortech, Warlock, too many more.
http://www.alternativeauto.com/
http://www.allmustang.com/
http://www.x-m-p.com/
Just go to Google/Yahoo/Etc, and type in Mustang Performance... you'll get hundreds of hits...