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I have some decorations that have standard fairy lights behind some plastic which creates a kind of holographic effect.
There is a similar product here: https://www.amazon.co.uk/dp/B07JB979HD
What is this material and what types of places sell it?
It's called "3D star effect foil" or "3D holographic foil" and can be found in well-equipped craft stores (especially around christmas time) or online: