Eastern Kingdoms
The starting areas of the Eastern Kingdoms are rich with veins of copper and tin. As you progress into higher level areas, you will find more valuable ores such as iron, silver, gold and mithril. Truesilver can be found in all level 30-60 zones, as well as the Hillsbrad Foothills. Dark Iron can be found in Searing Gorge and Burning Steppes. Thorium is available in the Hinterlands, Searing Gorge, the Blasted Lands, Burning Steppes and both Western and Eastern Plaguelands. The Twilight Highlands contain both Elementium and Pyrite.
Kalimdor
Copper and tin can be found in all low-level areas of Kalimdor, except for Teldrassil, as mining is not a skill known to the Night Elves. Generally, Kalimdor is not as plentiful in ore as the Eastern Kingdoms. Mithril can be found in Azshara, Felwood, Un'Goro Crater, Winterspring, Silithus and occasionally in Thousand Needles. While Winterspring and Silithus are sources of Thorium, no place in Azeroth is as rich in this ore as Un'Goro Crater.
Outland
As far as miners are concerned, Outland is synonymous with Fel Iron. Outland is the only source of this metal. It can be found in all seven zones of Outland. Nagrand, Blade's Edge Mountains, Netherstorm, and Shadowmoon Valley are all also sources of both Adamantite and Khorium.
Northrend
The frozen wastes of Northrend are where you will find cobalt, Saronite and titanium. Cobalt is found in Borean Tundra, Howling Fjord, Dragonblight, Grizzly Hills and Zul'Drak. Saronite is found in all zones except the starting areas of Borean Tundra and Howling Fjord. Titanium is available in Sholazar Basin, Icecrown, Storm Peaks and Wintergrasp.