We are building house and having problems finding 4 sets of exterior french doors size 18"x8'. We are located in Alabama. Anyone know where we could find these doors?

You should probably look for a custom door or wood shop in your area.

Another option to check is that Marvin doors and windows makes their units custom to size. This shape may possibly be outside their parameters but woirth checking into because of the quality of their three point locking hardware.