Vantage Plane Plastics is where I got all my replacement plastic.
www.planeplastics.com
Not cheap, required some heating to shape and trimming. Not an easy job but looked respectable when done. My advice would be to salvage as much of your existing plastic as possible because it fits and the screw holes are already in place. Repair cracks as much as possible, clean them, prime them, and paint them. They will come out ok. I salvaged 80% of my old plastic. It wasn't perfect but a whole lot better when done. Fitting new plastic is a pain. Can be done, but you'll work for it!
Good Luck!
Ed