door panels

I'd guess you can make your own. Match the masonite pattern from the old ones, and make sure to copy the rectangular holes in them as well for the spring clips used to hold the panel on.

Then once fitted then buy some thin 1/4" or so sheet upholstery grade foam, cut to fit the shape of the panel, then adhere it with contact adhesive. After that use some upholstery grade vinyl or fabric what have you - cut to fit larger than the panel so it can folded around the edges - again adhere with contact adhesive and use short staples to secure on the backside. Not sure how you handle the corners because the material will bunch up, maybe a good reason to use fabric that can stretch. Some experimentation might be wise before getting in too deep.

Another thought is to research for an automotive interiors customizing book that covers making door panels etc.