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.
Storm Peaks Eternal Fire Cave
In the northern part of the Storm Peaks, there is a small cave, to the east of the Terrace of the Makers. In this cave, you will find Wailing Winds. These mobs are not immune to fire damage, and they respawn almost immediately. They do have a slightly decreased drop chance of only 16% though. The exact coordinates of the entrance are: 62.5 41.6.
There is also a more open spot further to the south-east, near Dun Niffelem. The Seething Revenantss here have a drop chance of around 27%, which is a lot higher than the above-mentioned spot.
Farming Eternal Fire in Wintergrasp
While Wintergrasp is known for its PvP action, there are also quite a few great farming locations there. This Eternal Fire Farming Location in Wintergrasp is one of them. The Raging Flames here have a drop chance of 37%, being even higher than the two spots in the Stormpeaks. Though, you might run into some trouble with the opposing faction here.