You get them from a variety of sources, not just world bosses, but they are not guaranteed. Nodes, world bosses, delve bosses, dungeon bosses, adds, treasure chests, etc. Also, if you camp at delve boss, public dungeon boss, or hop from world boss to world boss, there is a timer that will preclude you from getting any quality loot if you kill too quickly. Its not very long, but it is there