When looking for PET films for electronics applications, you’ll typically want to grab biaxially-oriented, thermally stabilized PET (aka BoPET ). This type of PET is chemically resistant, and dimensionally stable at ‘higher’ temperatures up to about 150C.
The more common form of PET is amorphous and not heat-stabilized - you can print on it, but it will have a lower max temp (~130C) and tend to shrink and expand more during heat processing.
Definitely stay away from anything that says PET-G, these have an intentionally lower melting temperature for forming applications.
As for suppliers, mcmaster seems to have both BoPET and amorphous PET. I haven’t tried either, but the BoPET is listed as semi-clear white, so it might not be all that transparent.
A common brand name to look for is Melinex, from Dupont-Teijin. A quick google search brings up a few distributors, e.g. Tekra.
I hope that helps!