Orc Bones is a quest available after Orc Plains has been passed for the first time. It is part of the main quest line and becomes unavailable after it has been completed.
A single dungeon run is enough to achieve this.
Click on [Campaign] -> Select the [Orc Plains] -> Kill [Tauren Warriors] to collect [Orc Bone] x10. Tauren troops are brutish and powerful, but your strength frightens them.