Silver ore is a rare ore node spawn. Silver ore shares spawn points with tin ore, so farming tin ore can result in silver ore spawning in its place.
Silver bars are used for the Silvered Bronze transmog set from Blacksmithing.
The best place to farm it is in Hillsbrad Foothill. You can also farm it in both Northern Stranglethorn and Feralas.
You can also farm the following gems: