You can see from the screenshot above, that this surpasses the prices on any essence, in fact, it is a constant on most servers that this is the main dog. Now there are a few options you can utilize to pick up enchanting mats. You can use the Auctioneer Enchant Mats search (however mine seems to be broken for quite some time now...), but once again, and you should know by now, I don't always cover the easiest way to get things. Knowledge is power, and I want (and prefer) you to know and get used to the names, the items, and what and where they come from. Some addon's take all the "smarts" out of things, and at times, is okay. But this way you will be checking and learning, which is a positive step in the AH PvP.
So what we do know is that Greater Eternal Essence is disenchanted from many different items. I am going to focus on a few different ways (and you can go from there to find more, I just don't want this post to go 20 pages long). Mainly, I start with a search on Auctioneer for levels 48-55, as that seems to pull the best luck for me. The numbers might be a bit off (a level here and there), but this will get you started.
Upon the mouseover, your tooltab should show something to this effect. If you look midway down the lower window, you will see a "Disenchants Into:" and this is what you need to check before buying off the AH. Now this item has a 75% chance to "drop" 1.5 Illusion Dusts, and a 20% chance to drop the kingpin, Greater Eternal Essence. Is this a possibility? Sure it is. Is the the "golden" opportunity? Not really. We want focus on a 75% chance to drop Greaters.
See where I am going with this? You feeling me? Good.
In order to get the most Essences (any style, any level), you will want to push for Weapons. Let's check the AH again under the "Weapon" tab..
Now the reason why I listed this second is because, well, just to see if you are reading the entire post, and not skimming through. Seriously. You should check the weapon tab first, as this is where your most profit and least risk will come from. Sneaky, huh? Now let's glance at the tooltip:

Now you can use the "Disenchant Value (AucAdv)" if you would like, but it's not something I look for. I go off the odds. Now we are seeing the 75% for Greaters, which is what we want. Remember, Blizz has a RNG for disenchants (random number generator), so if you just buy one to "flip", don't expect to get them everytime.