Prayer is a skill that can be trained from buying bones at the general store in Skillz and burrying them. This skill is the second fastest to get to 99. Bones are sometimes low on stock and may be hard to get, however they only cost 1 gp each and come in noted form. It takes 163 bones to get level 99 Prayer.
You can also get bones from killing
Daganoth Supremes at Mod Island, or chickens at the Slayer Tower.
Whenever Godzhell resets itself or has a mass disconnection, the General Store will have the Ourg bones reseted.
Use this opportunity to buy tons of Ourg bones!