You can make your own 2'x4' ones out of 1x4s and Roxul r60. It is very simple to do. Lowes has Roxul rockboard (special order...last time I purchased was $55 per bundle which had 6 or 8 pieces per bundle). I used some 2", and also some 3" (r80 I believe). You get the Roxul, and measure for a frame to enclose each sheet, build a frame around it, and then get some cheap material I used Muslim. Stretch it across the back of the frame, and staple it. Then place your Roxul in the frame (it should be firm but not compressed). Once you have this done...get some Acoustically transparent fabric (go to a local fabric store, and find a fabric you can breath through and looks like you want)...lay the fabric on the floor or table, and then pur the frame on top of it face down. Cut the material so you have enough to extend down the sides, and a inch or so onto the back piece. Grab the middle of the piece of material and staple it to the rear...then go to the opposite side of the frame and stretch the material, and staple it. Then go to the top or bottom of the frame, and do the same as you did for each side. You then work your way around the frame stretching from the center out until you get to the corners. At the corners you can stretch and fold...then staple. Once you are done you can do as I did , and get a French cleat from a local hardware store, and attach it to the rear of the panel, and the other end to the wall. You can also make your own cleats by taking a piece of wood, and ripping it with a 45 degree angle cut then just cross cut the wood to the desired width of the cleat.
You can do these easily on a weekend once you have all the materials at hand.