I've been told that I can get sound insulator padding at Hone Depot or Lowes, but when I went and looked I found nothing that looks like it would work. All I got was the look when I asked employees there about it. You guys that have gotten it there before, what was the product called?
Not at HD, you can find it at Lowes. It is called Reflectix, its foil faced bubblewrap, made to insulate home and commercial ductwork. I have two rolls in my garage that I intend to use on the panels in the back of my car this winter. I also purchased a roll of insulation intended to cover waterheaters. I cut it to fit the floors of my car, I will put the foil faced side down over the top of the black underlayment.
I also have purchased two gallons of this product to coat the rear inner fender, floors, and wheel houses. It looks like it will make a huge difference in the interior noise level. http://www.quietcar.net/
As for the stuff you find at hardware stores, it's more of a peel-n-stick roofing material. Basically just a type of tar paper. But people complain that it smells, so alternatives are often researched.
The big ones these days seem to be SecondSkin
Edead
Brownbread
Fatmat
DynaMat
Quietcar/Quietcoat