Actually, the only color named Vert d'eau that I remember is from Spring of 2007.
You might be thinking of a color from 2010, and have the name wrong.. Maybe Papete?
EDIT: Oops.. Sorry! I'm wrong! Just saw a PF'r with a 2010 "Vert d'eu" You might start calling any and all of the shops listed here:
Reputable Stores Carrying Balenciaga/ Buying from Overseas or Via Mail
Check here to see all the colors and their names over the years.
Color Charts by Year - All in One Place!!!
The further back you need to go for your dream bag, the less likely it might be found squirreled away in a back room of a retail shop...and the more likely you'll need to search among re-sellers, ebay or bonanza.
Good luck!