PET is also called Polyesters, or Polyethylene Terephthalate. It has good flexibility that can be bent to fit constricted spaces, so it is ideal material for manufacturing flexible PCBs. Besides, PET also has high chemical and moisture resistance.
PET is generally 25-125 microns thick, which means it owns small size and light weight.
Note: PET has a poor heat resistance, the temperature mentioned in the form is instant temperature, so soldering is not recommended.
Adhesive Polymide (PI) and Adhesiveless Polymide (PI) are also common substrate used for flexible circuit board.
Check this link to place a PET board order: https://www.pcbway.com/flexible.aspx.
