The window decals on my GT were purchased from the two companies listed below. They also make flames. From what I have read on forums other people who have purchased decals from them have been pleased with their products.
EDECALS.COM:
http://www.edecals.com
STREAMLINE DESIGN:
http://www.streamlinedesign.com/
The decals are of excellent quality IMO, however, only time will tell how they will hold up in the long-term. If you keep your car in a garage or under a carport decal flames will last longer before you may experience problems. I believe
proper installation is the key to success. Of course, if you want to spend the money--1.5 to 3.5K--depending on the size) you might want to consider hand-painted flames if you have a shop near you that does this; it is my understanding that, for the long-term, these will hold up better than decals and can be waxed/polished. Just my two cents.
Good luck.