Only buy stickers from the national french ministry website, linked below. Other sites are often much more expensive, if they provide you with a valid sticker at all.
Stickers can be bought (also by foreigners) from the national French Ministry. The very well made and easy to understand website is available in English, French, German, Italian and Spanish.
A sticker called Certificats qualité de l’air = 'Crit'Air' is mandatory for all vehicles if they want to enter a French
'low emission zone' called ZRC (zones with restricted circulation) or
'emergency low emission zone' called ZPA (air protection zone in cases of high pollution events).
So far there are:
Look for more information under 'National Scheme' further down on this page.