I was bored so I did a little searching around for ya.
I found a couple things.
http://www.mustangokla.com/
I found stuff on this website for the 69. They were selling just the outer door skin for around $125. Thats without S&H and is only the skin.
Is the car completely lacking the doors or are they present but just bad shape. If they are there and depending on the shape of them you might be able to use just the skin.
Here.
This place had the same door skins for about $115 without S&H.
Another one... This one claims to have the plain door shells for the 69 mach 1 but you have to call to get a price quote.
Hope this helps ya out a little. This was just about 20 minutes of searching...