You have to buy them from certified dealers. It is just like the SVT
program that for has and is doing away with. If you go to saleen and roush's web sites, they should have a way for you to find certified dealers in your state. I know saleen does...not sure on roush though. If you already have a car, just buy the parts and upgrade it yourself. It will be cheaper in the end. But the answer to part of your question is that you have to buy one made because saleen and roush buy the stang from ford and then tweak it.
"When I know more, I'll be forthcoming. Or I won't be forthcoming, and I'll be honestly deceptive." - Les Miles