This link will open the official page with the points of sale for museum passes.

It is supposedly the English page but the official organizers sadly didn't bother to translate the French page (sigh)... You will nevertheless find GoogleMaps links for each place.

And the most convenient and easy-to-find place for you (which is not on their list!) is the newly opened (October 2019) kiosk at the very top of Champ-Elysées on the left corner facing the Arch of Triumph.