You probably have to order them from Ford. Ford makes OEM door skins. I am not sure if any other company would carry the Mustang door skins.
I am curious why you need brand new door skins for your Mustang so soon? Did you get your current ones damaged and dented from getting into an accident? Or do they have rust on them? I am curious because I noticed that some 99-04 Mustangs have rust bubbles under the paint on the bottom inside part of the door skins right where there is a fold in them. You have to get under the doors and look under the doors to see the fold on the inside bottom part of the back of the door skin. Does anybody in here have the paint bubbling problem on the bottom of the door skin on the back of the door skin where it folds? I noticed that the paint bubbles in some areas there and that the road salt gets lodged in this area causing the paint to bubble and rust to appear.