Look here, this is the first thing that popped up on google when I searched for 'home depot light diffuser' http://strobist.blogspot.com/2007/10...ights-and.html Scroll down a bit.

That said, no amount of searching home depot online led me to it. I'm guessing it's because they don't ship well? I know a lot of the light covers were cracked when we had to replace one. Maybe Thomas can get us a part number?