A few members here have fibreglass doors on their cars so might be able to provide details of suppliers. From previous threads here, Bull Motif were developing some a year or so ago but they don't seem to be selling them yet.
I may be wrong but when i asked ben whos making a few fibregalss panels, he mentioned that he had seen some knocking around jlh. May be worth a ring atleast.
Mighty Minor in Castleford was in the process of making complete front doors. I have a very early prototype set on my Minor but they took a lot of work to get up to a usable standard.
One of the problems with them is that fibreglass is thicker than steel so you run into issues where parts fit on the inside of the doors.
The weight saving is huge. I can lift my doors with one finger.
I do belive JLH bought the moulds but I am not sure.