Chlkilt got it right - that's the best way.
For iron, if you're high enough level, I really like the Badlands. There's a forge in the middle of the zone for smelting between runs, and there is also a lot of mithril when you get that high. If you're on the horde side or have stealth, Thousand Needles is pretty rich territory as well, but alliance tends to get ganked there a lot. Silver you can't really farm - it is a rare spawn of tin, so anywhere you mine a lot of tin you may occasionally see a spawn of silver.
Good luck