You didn't really say what you wanted to do with your car so I'm going to guess your talking about doing major changes like heads, cams or superchargers. If so I would check out a couple shops that sponsor the Chicagoland mustang club (CMC)
The club is: www.chicagolandmustang.com
Then the shops are:
A good tuner is: http://www.kurganmotorsports.com/
(They do all sorts of stangs contact Bob)
Sutton High Performance: http://www.suttonhp.com/
(they might only do newer stangs I'm not completely sure)
You can contact them either through the clubs forums or you can call the shops. Like I said you didn't make it to clear as in what you were looking to do so I'm just guessing its not bolt ons that you could easily do yourself like exhaust and CAI. So just check the websites and see what they offer.