These are universal stamps you can use anywhere in "developed" countries. When you request a certificate (birth, marriage,death, ) from a municipality, it is generally requested a stamped enveloppe. But buying a foreign stamp is impossible, This "coupon reponse" allows you to do so.
Generally available in post offices, in France can be also obtained from this url http://timbres.laposte.fr/http://timbres.laposte.fr/ (only in french)
Try the same in your country, if not, you could request them from France
Average cost= 1,30€ for a 20 gr letter (some fees more as usual) Payment by credit securised prepal system as anywhere now.