Looking for a source of vibration dampening sheets

Dynamat is popular with Car Stereo installers.

http://www.dynamat.com/
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3M makes some. Available at amusicdirect.com.

Mike Powell of Verastarr.com sells what you ask about. His product is named Starrdamp, and one side of it is copper which also shields from RFI.
He is currently having a closeout on his mass loading granite blocks which are another aspect of tuning and clarifying.
I have used both of his products in a few of my systems with fine audible improvement.
Herbie's Audio Lab also has great products which you can read about on Herbiesaudiolab.com. I use in a few of my systems and the product line is extremely broad, and addictive.
Both sites list phone contact numbers to speak with the owner. ("Herbie" is a nom de plume) Both are dedicated audiophiles who spend time with a caller.

Thanks everyone, you guys gave me a good start. I'll scope out the options you provided.