I built several baffles for my listening area using panels called "Sonolead", which is two layers of high-density fiber glass with a thick, flexible vinyl material in the middle (like a sandwich). "Sonolead" panels were designed for use on marine diesel engines, and they REALLY cut the noise. I purchased these panels, plus other sound absorption material, from a local marine supply store. If you have a good marine supply store in your area that carries engine and engine room parts, you can probably find "Sonolead" or something similar. If you don't have a local source, my local store ships anywhere. The name of the store here in Seattle is Hamilton Jet, and their Web site lists a number of sound absorption materials. The Web address is: http://www.hamiltonjet.com/noise/noise-vibrations.html
