Note: Motes of Shadow will not be included in this list as its usually cheaper to buy in AH than take the time to farm, this list is incomplete and will be updated with more information such as coords and ratings. This is just a list of mobs that have best drop rates on the particular motes.
Hope this at least gets you guys pointed in right direction.
Mote of Life
[Zangarmarsh][62-64][Starving Fungal Giants]
Mote of Water
[Shadowmoon][67-69][Enraged Water Spirit]
Mote of Fire
[Blade's Edge][66-68][Searing Elemental]
[Shadowmoon][68-70][Enraged Fire Spirit]
Mote of Air
[Netherstorm][68-70][Enraged Air Spirit]
Mote of Earth
[Nagrand][64-65][Tortued Earth Spirit]
[Shadowmoon][67-69][Enraged Earth Spirit]
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