It's a rare drop which you can get by killing monster with Avikan hunting weapons (they are easy to tell apart by the hunting spear icon where the elemental type would normally be). The higher the level of the monster, the higher your chances are of getting a Great Monster Bone.

While chances are low, they do drop at all levels, so even on a starting planet you have a small chance to obtain these bones when killing a monster with the appropriate weapon.

thanks