Agree. As mentioned already, they are standard 4 X 2 light diffusers available at any big box store by the florescent lightning section. They come in different styles. Click on link below for an example.
http://www.homedepot.com/s/light%2520difussers?NCNI-5
http://www.homedepot.com/s/light%2520difussers?NCNI-5