You can retrieve hearts of daedra during a daedra quest: Fragments of the Past.
Go Gamescom Day 1: Opening Night Live You can also find another Daedra Heart near Riverwood Skyrim:Daedra Heart - The Unofficial Elder Scrolls Pages Specifically: At forges, if the Daedric Smithing perk has been unlocked, a Daedra heart can be combined with ebony ingots and leather strips to make: All varieties of Daedric armor (armor, boots, gauntlets, helmet, and shield) The Elder Scrolls V: Skyrim at IGN: walkthroughs, items, maps, video tips, and strategies.
Daedra hearts provide that essence, and therefore are critical for making all varieties of Daedric gear.