Fel Iron Ore can be farmed anywhere in the Outlands (Burning Crusade).
Because of CRZ, Hellfire Peninsula can be incredibly overfarmed. You are best to head to the higher level zones, as many skip those zones in favor of Northrend for leveling.
This is also the first area where nodes can be found anywhere in the zone, not just around mountains or rocks. As such, randomly flying through the zone should yield a high amount of ore.
A huge number of gems, stones and elementals can be farmed while mining fel iron, as well as Eternium Ore.