To start off, I am going to say that Wintergrasp is by far the best spot to farm any Eternals. This is so widely known at this point I am still going to show you the spots on where to farm for what Eternals. But because of it being so widely known as Wintergrasp being a farm spot, I am going to show you some other spots for farming Eternals with high drop rates and hopefully the competition won't be as heated!
Eternal Air:
In Wintergrasp, your best bet will be the Tempest Revenants and the Whispering Winds. In the general southern area of Wintergrasp.
The Scions of Storm located by Thunderfall is an alternative in case you don't want to deal with other people.
The Storm Revenants behind the Maker's Perch in Scholazar Basin. This place will also be a viable source for Eternal Water which you will learn later in this guide.
Eternal Earth:
In Wintergrasp you will find Chilled Earth Elemental and Earthbound Revenant in the top right hand corner of the map.
The Lifebood Elemental's in Scholazar Basin that are around Lifeblood Pillar is a good alternative spot from farming in Wintergrasp.
Another good spot to check out would be the Brittle Revenants in Storm Peaks, at Frostfield Lake.
In Zul'drak, there is a decent spot by Ebon Watch where you can kill Icetouched Earthragers as they have a decent drop rate.
Eternal Fire:
In Wintergrasp, the best source of income for Crystallized Fire are the Flaming Revenants and the Raging Flames in the very far bottom right hand corner of Wintergrasp.
You will find 2 good spots in the Storm Peaks, as the Wailing Winds (near the Eternal Air spot) and the Seething Revenants at Fjorn's Anvil. Both spots have high drop rates for Crystallized Fire.
Eternal Life:
The Living Lasher's in Wintergrasp will be your viable source of crystallized life there, towards the bottom left hand corner of Wintergrasp.
Mossy Rampagers in Zul'Drak is the next best option, the spot is located just northeast of The Argent Strand.
Eternal Shadow:
In Wintergrasp, You will find Shadow Revenants and Wandering Shadows as you will mainly want to focus on the Shadow Revenants as they have a lot higher drop rate although you should still kill the Wandering Shadows as they have a good drop rate also. You can find both of these towards the bottom left hand corner of Wintergrasp near the Eternal Life farming spot.
As a decent alternative spot for Eternal Shadow you can kill the Deathbringer Revenants at Frostmourne Cave in Dragonblight for a decent amount of Crystallized Shadow.
Eternal Water:
The Glacial Spirits and Water Revenants are a good source from Wintergrasp, they are generally located in the Lakes.
Icebound Revenants found in the Storm Peaks are located at the Frigid Tomb is one of the best alternatives for Crystallized Waters.
Aqueous Spirits located in the same area as the Eternal Air spot is another good spot for Waters and I would highly recommend if you are in it for gold as you will bring in two types of Eternals.
Maps: