This Crystallized Life farming guide will show you where I farm Crystallized Life in Wrath of the Lich King Classic.
Eternal Life comes from combining 10 Crystallized Life, so if you need Eternal Life, you have to farm Crystallized Life.
Living Lashers in Wintergrasp have the best Crystallized Life drop rate, but the respawn rate is slow. If you play with a class that can kill them fast, you will always have to wait for respawns.
Thornvine Creepers have a lower drop rate than the mobs in Wintergrasp, but the respawn rate is really fast, so you can never run out of mobs. They are also low levels mobs, so you can kill them much faster.