If you read any of the Mustang oriented magazines you will see a large variety of companies selling Mustang parts and accessories. As mentioned earlier American Muscle is by far the most popular choice among our members as they ship fast and do not charge for delivery, a huge plus!
That being said there are some other great companies that may offer more than American Muscle such as Mustangs Unlimited, CJ Pony Parts and Moss Mustang parts. I have made a lot of upgrades to my car, a 2002 Mustang, and American Muscle has been my supplier of choice for over 90% of my purchases.
They also have a great interactive website that describes the installation of these accessories but if you need someone to install anything than you need to talk to Bob Thompson. He has installed a lot of accessories on my car, including white faced gauges, new vinyl striping, stainless steel door sills, hood struts, and fog lights.
Midnight Blue
MCOM Treasurer-Historian