Check your local hardware store, they should have various sheets in different thicknesses. If no go, the web offers everything.
The typical sheet quan***y 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.
In my shop we used 3M Ribbon Sealer, part #8610. Bought it from my local Napa store. Looks like strip calk but MUCH stickier.
What mil visqueen are you looking at, It comes up to 100 mil, finding it in small quan***ies might be a issue. Check some construction supply houses not the box stores.
I used clear shelf contact paper from the local hardware store. (It’s used to cover shelves in cupboards. Comes in all sorts of patterns) It’s thick enough and...it’s adhesive backed and sticks just enough. I removed an inner door panel this summer after installing in 09’and it still looked just like it did when installed.