These are stickers made by Quantum Mechanix (Qmx). They're sold in a pack of two different stickers. I bought a bunch of them when allposters.com was selling them on clearance.

They used to be available on Amazon, but I haven't seen them in a while.

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This is what the stickers look like, but these are NOT the original high-quality metallic decals. These are copies that someone is printing, and they're not the same.

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*EDIT* Ah, found the original Qmx versions on Amazon, but they're unavailble.

https://www.amazon.com/Firefly-Builders-Plaques-Metalized-Sticker/dp/B00506WNQI?ref_=ast_sto_dp