Asking for light diffuser should help, as they generally have an isle full of different types. Be aware that Home Depot stocks two different brands of the white eggcrate, and one is smaller than the other but around 50 cents more expensive. IME, it is located on the isle with the fluorescent tubes, fixtures, and the shop lights. If they still don't know what you are talking about when you say "light diffuser" just tell them it is the panels that you put under a flourescent light to keep the glare down, maybe they'll know what you are talking about then.
Hobby Lobby would be very unlikely to carry it, lol. I bet we're pretty much the only hobby that uses it (I'm sure I'm wrong though, lol). Heck, I'm waiting for one of the stores in the mall to close again so I can go pick through the dumpster to get all the peices they were using for their lights.
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