Owens Corning 703 Acoustic insulation works very well. OC 705 is thicker and works for the lower resonance. You can get these at most HVAC or Drywall businesses. They are not sold at the big box stores.
Cover them with any color Burlap material (can be purchased from Wal-Mart) to make custom sound panels. Companies like ATS Acoustics make these, if you want to see what these panels look like at retail.
You can easily fit them to your own needs for a fraction of the cost. Lots of DIY instructions are available on home theater forums.