.... but, of all the projects to scratch that's about the easiest.

Get the aftermarket (they are usually free) 1:1 Catalogs from Classic Industries, Year One, etc. They will have standard Impala trim interior pics to sell through their repro interior bits.

Then you get out the drafting board and go to town, transfer the pattern to the substrate of choice, and make your own. Trace the shape and pattern onto the sheet evergreen, then anything you need to get the pleats/rolls are fair game. Many patterns can just be scribed. but if there's something you like better, do it.

At first plan on making a lot of scraps. But as you get the hang of it, it's pretty fun.