Metal Ores can be found throughout Middle Earth. Metal Ore can be collected by Prospectors, who can also smelt the ore into metal ingots, and in the case of iron, refine the ingots into various sorts of steel.
Metal Ore comes in "tiers" of increasing difficulty to mine. Only sufficiently experienced Prospectors can mine the more advanced ores. The Tier of the ores found corresponds roughly to the terrain and monster difficulty, with higher Tier ores found in higher level and more dangerous areas.
Tier 1 Copper, Tin
Tier 2 Barrow Iron, Silver
Tier 3 Rich Iron, Gold
Tier 4 Dwarf Iron, Platinum
Tier 5 Ancient Iron, Misty-Mountain Silver
Note that in some areas you may find ores of different Tiers mixed in the same region - for example, the Trollshaws spawns Dwarf Iron and Gold.
Ore Nodes have fixed locations in the landscape and are populated randomly by the terrain generator after they are removed.
In most of the areas, the iron ores are more common than the precious metal ores. However, selective removal may make the apparent ratio of iron/precious seem very skewed in either direction. If the ore you are looking for seems very rare, try removing the other ore nodes in the same region and wait for them to respawn - this will generally give more of the rarer ore than simply wandering the landscape and hoping to get lucky.