Sorry to be pedantic, but while it's entirely true that many things are called by the wrong names, real clear cellulose acetate sheet is still widely available, as a google search of the term "clear acetate sheet" will show (clear sheet made from PETG, styrene, acrylic plastic, etc. is usually correctly labeled and marketed under the correct name of the material it actually is as well).
Genuine cellulose acetate clear sheet is even marketed for specific aircraft uses, and in aviation, you can't call stuff by the wrong name.
https://www.aircraftspruce.com/catalog/mepages/acetatesheet.php
Here's just one of at least hundreds of commercial sources.
https://www.jacksonsart.com/en-us/grafix-clear-acetate-pad-11x14in-003in-25-sheets