The [armory]Bones of Eao[/armory] are a random drop from different activities in Destiny, like the Raids or Nightfall. It's also something that can be sold by Xur. I've attached to the name a link to its armoury entry for your convenience. I've also linked the page itself above. As it says, Nightfall, Raids, and Xur are all ways to get it, though Exotic Leg engrams also have the chance.