mylar

3, 4.2 and 6 micron available in The Netherlands.
2, 3 and 6 micron available also through Ebay from Taiwan. The 2 micron offered here is actually 3.0 micron and the 3 micron offered is in reality 3.5 micron. Both claim to be original Dupont Mylar.
 
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See if searching for "polyester film" produces any better results.

Hostaphan is another trade name for a different manufacturer's polyester film. Sometimes that's easier to find locally than Mylar.

Small amounts (less than miles) of thin film can be hard to find from typical plastic suppliers though. The earlier suggestion of E-bay is a good one. ER Audio may be another option if you can't find anything close.