Sites such as wow.Allakhazam.com and wowhead.com can help you out a lot here, they have detailed maps with ore locations.
If you open up wowhead and do a search for (for example) copper vein, this results in a listing of zones, along with a given amount of ore nodes present in that zone. You can also see a map with ore locations.
Allakhazam uses a similar process. Doing a search for a vein or deposit type will list all of the zones it is found in as links. Clicking the link will display the map for that zone with ore locations. Every square represents an ore location.
Only a given number of ore veins can be up at any time, so don’t expect to ever actually see the amount of ore veins shown on these maps spawned simultaneously ingame.
To make searching easier, it is important that you type the name of the ore correctly.

Ore Node Names