I've been buying them from the Alchemy shop in Whiterun. Just keep checking every 2 - 3 game days until they appear. --- Current Playing: Wipeout Series (PS1, N64, PS2, PS3, PSP), Forza Motorsport Series (Xbox, Xbox 360).
If you havn't just do the tiny magic trick for gaining entrance, second floor Hall of attainment, enthir has 2 that he restocks every 2 days without fail, that is where I typically get all of mine --- I shoot you ,you die, you shoot me I die woohoo
do the mythic dawn quest in dawnstar. if you do it 4 dremora will appear outside dagons shrine and they respawn about every week --- The legend will never die. RIP Halofan: 3/19/12 http://www.gamefaqs.com/boards/615803-the-elder-scrolls-v-skyrim/62287165
do the quest for Mehrune's Razor (you will get a pamphlet about a museum filled with items from the Mythic Dawn, an evil guild in TES IV: Oblivion), once you get to the Shrine of Mehrunes Dagon you will have a few Daedra to fight outside and a few inside (just be good at fighting and have high combat skills since Daedra are tough)
Alternately (and the most easy technique that i have found) is to sneak and take the heart on a plate in the room Kodlak is in in Jorrvaskr in Whiterun. Either that, or use invisibility or become head of The Companions and take it with no crime. Also, this heart respawns every 2-3 days i think
Or simply buy it from an Alchemist, they will all have one eventually, but they are expensive --- XBL GT: iamRevan117
i like how each of the last three comments were just more progressively elaborate versions of the same thing --- The legend will never die. RIP Halofan: 3/19/12 http://www.gamefaqs.com/boards/615803-the-elder-scrolls-v-skyrim/62287165