Here dress is the Luna V-back dress by Elizabeth and James (so I found out here):
This amazing dress is sold several places, however seems to be sold out right now....
Net-a-porter ($262.50)
Neiman Marcus ($187, but in emerald)
Shopbop (not listed)
Get it now....Ebay ($149, in emerald, size 2/4)
Similar and for less options:
Urban Outfitters ($30)
Asos ($145)
Shopbop ($71)