This Crystallized Earth farming guide will show you where I farm Crystallized Earth in Wrath of the Lich King Classic. Eternal Earth comes from combining 10 Crystallized Earth.
The Storm Peaks
Brittle Revenants in Strom Peaks are a good source of Crystallized Earth.
You have complete the Sons of Hodir intro quests to unlock this farming place otherwise you will see a bunch of level 80 elites patrolling this area.
Lifeblood Elementals in Sholazar Basin are also a good source of Crystallized Earth. The respawn rate is fast, so you can farm here without downtime.