After starting Zogre Flesh Eaters, you can loot ogre coffins by using ogre coffin keys on them. You'll often get random junk (bronze equipment, damaged armour), but about half the time you'll get zogre bones, and 10% of the time each you'll get raurg bones or fayrg bones. Ourg bones are especially rare, with about a 2% chance. Most of the profit from this method comes from the raurg bones, which are used for Daily Challenges and are far more expensive than any of the others.