I got mine from American Muscle. $569 shipped I believe, and they will send you out the custom tunes if you sign up for the custom tune package, and tell them your mods. SCT Mustang LiveWire Custom Tuner (96-10) w/ 3 Free Custom Tunes at AmericanMuscle.com - Free Shipping!
First time I dealt with them, but I will be using them a lot more, as my wife actually ordered me the wrong tuner, and they took care of it right away when I called with 0 hassle. I have seen other positive posts from them since I joind this board too.
If you are looking to only purchase a tune, and not a unit, then I am not sure about pricing on that for your unit.
PS. The tune I got the lean code on was not a AM/BAMA tune it was an SCT preload tune... My Custom AM/BAMA tunes are great, and each is tuned for the purpose they say. The race is actually too agressive for daily driving around here, I had to brake constantly to keep from rear ending people in stop and go traffic, but it was fun to play with and would be awesome on the track. I use the performance tune most of the time.