Eternal Water is not quite as much needed as the other elemental materials, although Blizzard has tried to even out the need for these in Wrath. As always, they can be created using 10 Crystallized Water. (Some might call it “Ice”)
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.
Eternal Fire is one of the most sought-after crafting materials in Wrath of the Lich King. It can be acquired by combining 10 Crystallized Fire, which drops in amounts of 1-2 from Fire Elementals around Northrend.