Former Saleen owner (and current tender of the car) here. When the car is registered, it registers and insures as a Ford Mustang GT, which is what it started life as.
I got mine from Performance Autosport, the last link in the above post. The guy who owns PAS is super-nice, really knows his cars, and has probably bought and sold every model of Saleen ever made. He also does servicing, but most owners either do service themselves or get a trusted speed shop to do it.
A little Saleen history: Saleen was started by Steve Saleen back in the 80s. Saleen has modified not only Mustangs, but Explorers, Focuses, and Rangers. The modifications range from body kits, wheels, and dash gauges, up to superchargers, nitrous, and custom Saleen colors. The taken off parts were sold to whoever wanted them. Each auto was given a Saleen number, which is displayed on the dash, front left bumper, and engine bay. There are “Faleens” (fake Saleens) out there, it depends on how good a job you’ve done if you’ll be mocked or not.
However, Saleen was sold to a group of investors, and now Steve Saleen has his own company, SMS Supercars http://www.smssupercars.com/. He’s a really nice guy, too, I got to meet him at the 45th anniversary celebration for the Mustang in 2009.
A great resource is Brad Bowling’s Saleen Book. It has tons of information about the cars, and has a listing in the back of all the Saleens built in the first 20 years.
Also, the Saleen Club of America has guys who can get super-pedantic about what is and is not correct on your car.
I could post all night about this–anything else you want to know, just ask!