The motes and primals of Air, Earth, Fire, Water, Life, Shadow and Mana are needed by all of the crafting professions from level 325 and up. These can also prove to be a great source of making tons fo gold!
Before farming motes there are some things to keep in mind:
One of the more popular spots for farming motes in the Elemental Plateau in Nagrand located at in the northeast of the zone. A flying mount is required to reach it. Since this is a very popular place, farming here can be very competitive. You can find Water, Fire, and Air motes there.
Motes of Water
There are several zones to get these. My easiest monster to farm is the Shimmerscale Eel in Silmyr Lake in northern Terokkar Forest. These guys are only level 61-63, so they fast, and their drop rate is good. The best zone for farming Motes of Water is probably Nagrand. There you can find water elementals in just about any lake.
Mote of Earth
This is probably the cheapest mote of them all, because they are so easy to get when mining. The best zone for farming Motes of Earth is Nagrand. There you can find earth elementals all over the place. The best drop rate is from Shattered Rumblers of which are south and west of Oshu'gun.
Motes of Fire
The best spot for Motes of Fire is along the western border of The Hand of Gul'dan in Shadowmoon Valley. Here you'll find lots of Enraged Fire Spirits. Another good spot is Skald (i.e. east of Gruul's Lair) in the Blade's Edge Mountains where you'll find Searing Elementals.
Motes of Air
The best area for farming Mote of Air is Shadowmoon Valley. There, you'll find Enraged Air Spirits in several locations. One is the Netherwing Fields to the southeast, with a large concentration in Netherwing Pass to the south. The smaller area is north of the Altar of Sha'tar. In Nagrand, you can find low level air elementals scattered all over the zone, but in no large concentrations.
Mote of Mana
The best place to farm Motes of Mana is Netherstorm. There are two good farming spots there. The first is Kirin'var Village, where you should target Mana Seekers and Mageslayers. The second is the Ruins of Enkaat, where you should go after Mana Wraiths and Nether Rays. If the Ruins of Enkaat is too busily farmed, you might want to go a little further northwest for the Phase Hunters.
Motes of Shadow
Farming Motes of Shadow is relatively light on competition. The Voidspawn wandering the area around Oshu'gun in Nagrand is probably the best spot. Just run circles around the area killing them as you encounter them. Its probably one of the easiest motes to farm.