Looking for a source of vibration dampening sheets

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.

I don't think this will do what you want, but it is so inexpensive, you ought to look at it. At the local hardware store, you can buy sorbathane peforated mats, which go in your kitchen drawers and cabinents to keep things from sliding around. They are very flexible and come in rolls and are very inexpensive.

Stack them up, layer by layer, to kill any vibration you want to kill. Cheap and works great.

The only catch is, they aren't adhesive, so if you want to keep your chassis from vibrating, it won't do the job.

Partsexpress.com has great selection and sizes, and they are speaker and home audio based sheets for the actual purpose, used in many hi-end components, they also carry dynamat with the metal backing that I found very good as well, but you could just go to best buy and get that, it is all very cheap from parts express however at about 1.50 10" X 10" sheet.