Thank you for your e-mail.
As a matter of policy we do not recommend individual companies or products. Companies I know manufacturing doors and other similar products with similar volumes to yours often use vacuum forming methods. There is also some companies who prefer panels made from GRP.
What I suggest you do is to look at Kelly's Industrial Pages specifically in the sections relating to Plastic Mouldings, Plastic Vacuum forming and glass fibre mouldings. You will also see there are also machinery suppliers listed. Kelly's is best used in hard copy - it is a mighty tome - but you can access via the web on www.kellysearch.com. Most librarys have a copy of Kelly's. I don't know where you are located but you should find companies near to you that you can talk to.
You can also look on the web for door panels and in the window trade magazines where you will find information referring to the well known companies marketing door panels in volume.
I hope this helps.