Buying Big Bones off the G.E. is the fastest way, certainly. But the Chaos Temple, at level 13 wilderness, has lots of regular bone re-spawns. Although they don't give as much experience apiece as big bones, they're free, and there are enough bone re-spawns at the temple that by the time you manage to pick up all of the bones, more will have appeared. Plus, the only revanent that actually goes inside the temple is a level 105 Revanent Ork. So unless your combat level is 92-118, you have absolutely nothing to worry about. Also if you want free big bones, there are free ones at the bone yard in the wild. just pick up the bones and bury them, then wait for them to respawn. there are few skeletons wondering around so if you do wish to stay, bring some food.