I had to replace the plastic in the doors of the Coupe in My sig.
I too couldn't find it to buy anywhere, and then, It occurred to me while taking a shower one day, the Plastic shower curtain liner was the correct thickness.
So I went to Target, and bought the thickest
shower liner I could find. IIRC, I paid like 10 bucks or so for one, and it was large enough for both doors.
I spread it out on the floor and used the door panel as a template. It worked great. I cut it larger than it needed to be, and trimmed the excess with a razor knife after putting the panel back on.
Here's a tip,
do not cut a hole for the door speaker, It fits without it. Good Luck!