The Elemental Plateau in Nagrand lies north of the Throne of the Elements. It is only reachable with a flying mount. You can find level 70 and 71 elementals of all types here, as well as Pure Water fishing spots.
Fire elementals can be found in the charred region in the center. Water elementals are in the western part of the plateau in and around a lake. Air elementals are on the most eastern side on a grassy shelf. Earth elementals in small pockets in between, and the outermost cliff.
In patch 2.0.7, the spawn rate of mobs there was very highly reduced and can be frustrating if there's more than one person farming.