Seek Plastic Fluorescent Lamp Cover

I am seeking Plastic Fluorescent Lamp Covers. The frame is 23" x 23". See picture. http://www.angelfire.com/pro/young707/post1.html
Please tell me where to buy it in Northern California?
I do not find it at HomeDepot, Lowes, or Orchard Supply or WalMart.
Many years ago, I found some available at some plastic retail store in Dublin. But they are gone.
Thank you. (I Notice my NNTP server blocks attachments).
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Just like it is supposed to!
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