After Eternal Fire, Eternal Air is the second most used elemental crafting ingredient in Wrath of the Lich King. You can create it by combining 10 Crystallized Air, which drop from the various spots listed in this post.
Farming Eternal Air in Storm Peaks
There are two spots in Storm Peaks where you can farm Eternal Air. In the west, there are Stormriders, with a 31% drop chance. In addition, Further to the east of the zone, there is the Scion of Storm, which has a drop chance of 30%. There are quests for both of these locations, though.
Elite Altar Wardens in Zul’Drak
The Altar Wardens in Zul’Drak have around 36k Health and have a 33% chance to drop between 2 and 4 Crystallized Air, instead of 1 or 2. While there aren’t a lot of mobs here, there is only one quest that requires one of these mobs. If you are alone here and can kill them, this is a great spot.