Check your local hardware store, they should have various sheets in different thicknesses. If no go, the web offers everything.
The typical sheet quantity is too much for a car interior. Look for super HD trash bags, drop cloth (plastic) or other smaller amounts.
I got some from a fabric store. They had it in a couple of thicknesses on a roll. You could buy as much as you needed.
Home Depot. They have plastic sheet from 1 mil thick (paper thin) to 6 or 7 mil (which is much thicker). Plus, you'll have enough to do a half dozen cars for very little coin........ https://www.homedepot.com/p/HDX-10-ft-x-25-ft-Clear-3-5-mil-Plastic-Sheeting-RSHD3510-25C/204711653
As mentioned you can get many different thicknesses of plastic at the big box store, but does it really matter? All it has to do is provide a moisture barrier, and once it's installed and the inner door panel is in place it's not like it's in danger of being damaged.
I went to a local furniture store and the manager gave me the plastic sheets that mattress' come in. It was thicker and worked great. And, it was free.