Whenever I want something that originally came with my chassis, I contact the manufacturer. I have a Thermaltake chassis, so I emailed them about some stock parts, like side windows and such. They do have them, but it depends how old your case is.
Bottom-line, go to your chassis' manufacturer's website, get the contact email (don't bother calling...they won't be able to transfer your request most of the time) and send a request for whatever you need. Most cases have their blank bay panels designed differently. I know mine are.
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