Brian:
Home Depot work shop floor mat material.
Found in the carpet/flooring department. Comes on a roll of 48" wide and they cut off what you want. It's usually black, and has a smooth side and a shallow ribbed side. It's about 3/8-7/16" thick and is open-celled foam material.
I used 3M super heavy duty upholstery adhesive to mount it; two good coats on the mat, two coats on the inside of the door, let them sit for 5 minutes to tack up and then sitck 'em together. You can pull and re-apply maybe twice before it sticks for good.
I used it inside of the doors, under the carpeting, under the rear seat area and as a firewall pad. The stuff really quiets things down.
gn