Essence of Air is a crafting material used in a few high level vanilla formulas, including heirloom weapon agility enchants. Since they're rarely obtained while leveling, they're often in short supply on the auction house. Here are a couple of ways you can obtain Essences of Air to use for crafting or to sell on the AH for lots of gold!
Farming Essence of Air – Northwest Silithus
In the northwest corner of Silithus, you will find a large population of elementals. The wind elementals have about a 5% chance to drop an Essence of Air. By the time you've cleared all of the wind elementals, the ones you've killed first should soon start respawning again, making this a very profitable farming spot. In addition to Essences of Air, you'll also get lots of greens, and even the gray vendor trash adds up.
Transmuting Essence of Air from Essence of Water
If you're an alchemist, you can purchase a formula from a vendor outside of Scholomance that will allow you to transmute Essence of Water into Essence of Air. This shares a daily cooldown with other transmutations. If you are a transmutation master, you will have a chance to proc additional Essences of Air when doing this transmute.
Since farming Essence of Air is a lot more profitable use of your time than farming for Essence of Water, a good strategy is to check the AH regularly for cheap Essences of Water and pick them up whenever available.
How to Corner the Market on Essence of Air
Since this is usually not a commonly farmed item, it makes a great gold making niche, especially if you're an enchanter. By checking the auction house regularly for cheap Essence of Air and Large Brilliant Shards and farming the rest yourself, you can craft agility heirloom weapon enchants on the cheap. (The agility enchants are obtained with Timbermaw Hold reputation.)
Check The Undermine Journal to research the market prices on the essences, large brilliant shards, and agility weapon enchants on your server. This will help you determine what is a reasonable price to pay for these items when they appear on the AH. You can also make gold reselling the materials if you've got a healthy supply on hand. This allows you to make some gold even if the buyer is going to have someone else craft the enchant. By picking up cheap crafting materials regularly, you deny your competition the opportunity to compete with you at a discount!