Motes of fire, air and water drop off the appropriate elementals. The most convenient place to find them is on the Elemental Plateau of Nagrand. It is so convenient a place, however, that you may have to compete with other players for the spawns there.
As an alternative, try the air elementals in the mountains of Shadowmoon Valley for motes of air and the water elementals around Haala in Nagrand for motes of water. For motes of fire, try mining lots of Fel Iron.
Motes of earth drop for miners all the time. No point in farming them...if you're not a miner yourself, you can buy them dirt cheap on the AH.