i've seen them around from time to time but i dunno where or price. I think here in Nebraska they are technically legal but cops are trying to make them illegal for the obvious reason of obscurring your lic plate #. i personally dont really like the look of them i would rather have a nice plate frames.
I saw a Mythbusters on both of these products and some other ones. And none of them worked on speed cameras, but the cover that distorts the letters from the side slightly work but not enough to get away free. Then again maybe they did this to so people wouldnt buy them?
i watched that same episiode of mythbusters...they tried the spray, the diffrent kinds of plate covers on the market and none of them worked...the best thing to do is go purchase an antenna motor, and some spring loaded hinges for your liscence plate.
wire the antenna motor and connect it so it pushes the top of the plate to fold it down, and use the spring loaded hinges on the bottom so the plate can fold down and spring back when you switch the antenna motor to close..its really not that hard to do.