I don't know of anyone buying them out fstburn. The basics of any good door panel is using a panel board that will not deteriorate with moisture and other elements. The best I have found to use is PVC sheets. The last I bought were approximately 1/8 inch thick (3 mm). they can be sewn, glued, and formed with some gentle heat. Best of all, they are reasonably priced. I believe I gave 22 dollars for the last 4x8 sheet I bought about 2 years ago.
DanTwoLakes has some great tutorials that will provide a wealth of information for you also. I also really liked the Sid Chavers DVD on panels. Very well done panels can be made with a little foresight and initiative even if you don't sew.
Good luck, We like pictures