Anything that insulates the car’s interior from the exterior can be used as car sound proof material. There are a few companies that make sound proofing materials for cars. However, the one company that most car enthusiasts, restorers and modifiers use the most is Dynamat. Dynamat carries a full line of sound deadening materials for any type and area of a car. Dynamat carries products in a variety of thicknesses, so you’ll be sure to find something for that hard to reach spot in your car. Most of the parts they sell come pre-cut to fit in specific areas.
For more info, see Which 12″ Car Speaker Model Is the Best