I am looking for the 1968 French departments administrative boundaries. I've scoured the Internet but have had no luck.
JPEG from 1964: Library of Congress
You will have to georeference the image and digitise it yourself.
You can use "Historique des commune" from insee (https://www.insee.fr/fr/information/2114819#titre-bloc-11) which contains all modifications in french city since 1943.
It doesn't contain boundary, but can help to check the evolution of these administrative set.