What level mining are you at? Low level ores are found in all the starting areas, i have found the barrens to be pretty badass for copper and tin. Elwynn forest also if your allinace, but there is no tin there you would have to go to westfall or redridge.
High level ore... There is a good farming spot in Arathi where the nodes respawn every 15 minutes, its right behind stormgarde keep through that little canyon. Thorium: best place i have seen is in the hives of Silithus, also winterspring and EPL.
P.S. There is a map at the bottom left of this page to show you where possible nodes spawn for each mineral. Hope that helps