Hi,
I am an American who retired in Papeari, Tahiti. You can buy the Serpent d'eau stamp online from the OPT here in Papeete. It is 114 Pacific French Francs, about 1.50 Canadian dollars. I don't go to Papeete every week, but if you send your address, I will see if I can get one and send it to you.
Steve (Manarii Tane, my Tahitian name)