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All reality is a game. Physics at its most fundamental, the very fabric of our universe, results directly from the interaction of certain fairly simple rules, and chance... (Iain Banks)
More specifically (the white example you're holding) it's bonded polyethylene foam sheets, likely 3 x 3mm sheets.
Very commonly called expanded polyethylene, or EPE. I get as much as I want from the IT people at work - it is used to pad electrical components.