This Crystallized Air farming guide will show you where I farm Crystallized Air in Wrath of the Lich King Classic. Eternal Air comes from combining 10 Crystallized Air, so if you need Eternal Air, you have to farm Crystallized Air.
The Storm Peaks
Farming Scion of Storms in Strom Peaks is a good source of Crystallized Air.
Enraged Tempests have a bit lower drop rate than the mobs in Storm Peaks, but they are low level mobs that you can kill quickly, and you will never run out of mobs to kill because they respawn quickly.
Altar Wardens in Zul'Drak are another good source of Crystallized Air. These are elite mobs, so they are harder to kill, but they often drop 3-4 Crystallized Air.