I did not find any in Brooklyn Home Depot MANY TIMES I was there. BTW I think the Home Depot in Brooklyn is the worst of all. I did find some in Staten Island last time I was there.
You need to go to the section where they sell drop ceiling tiles. It is actually called light difuser, and you may need to describe it to them. It will be buried on a shelf more than likely, and they will not even know they have it.
It took me a LONG time to find it in the Home Depot in Yonkers. Finally found it and the box was covered in a lot of dust.
I've gotten it at Home Depot and Lowes. Its not always in the drop ceiling section. Sometimes it is in the fluorescent tube fixture section. It's with the commercial fixtures not the residential, pretty, fixtures. It is however always in a box covered in dust, often behind other stuff.