Find an Amulet of Akatosh

Daedra hearts are a rare drop from Daedra mobs: Atronachs and Dremoras. They will not drop from conjured Daedra, though: so no farming your conjured servants. Good places to find Daedra are during the Daedric quests The Black Star and Pieces of the Past.

In addition to that:

• One can be stolen from the Nightcaller Temple in the Daedric quest Waking Nightmare
• One can be stolen from Kodiak's room in Jorrvaskr in Whiterun
• Enthir in Winterhold College sells 2 every 48 hours at a huge premium
• They will rarely show up for sale on random vendors throughout the world

The Wikia suggests a few other places as well:

• Alchemists in Orc Camps will sell 2-3 of them
• Babette at the Dark Brotherhood Sanctuary
• One at Hall of the Vigilant directly south of Dawnstar and near Red Road Pass where a giant exists
• Random chance to be on any of several vendors in Markarth Side

The giant golden globe roughly in the centre of Blackreach is not the lexicon machine, but it provides a useful point of reference to find it. While standing below the giant golden globe, walk directly south. As you pass through the southern gate of the wall, you will be on top of a set of stairs. Slightly to your right is a tall tower that connects to the ceiling. This is the Tower of Mzark. To reach it, you will cross a bridge over rapids which flow to the edge of a waterfall. When you reach the tower, take the elevator to the tower's interior, solve the puzzle, and you will be able to transcribe the Elder Scroll into the lexicon and then retrieve the Elder Scroll from the machine.