We managed to find a lot of Iron Ore early on in Monster Hunter Wild's Plains environment. If you open up your map and zoom into the world, you'll spot rock icons that say "Mining Outcrop." ...

Geologists uncover the largest iron ore deposit... its value is... it is in... A newly discovered iron ore deposit has been identified as the largest ever recorded ...

While you can find plenty of resources by exploring the open map of Wilds ... find many Mining Outcrop. Mining from them has a random chance of yielding Iron Ore, along with other materials.