Could I have some people list some locations? I've been playing for hundreds of hours and found only three hearts. I want to forge Deadric armor please help!!!

Here's a list with all locations:

http://uesp.net/wiki/Skyrim:Daedra_Heart

They are rare. What I do when I really need more than 2 is go to the College of Winterhold, buy the two Enthir has and then just wait 48 hours for his inventory to refresh.

Also, if you get the Merchant perk in Speech, you will have the side effect of alchemy vendors selling the rare ingredients, including hearts. They will still be rare, but you'll find them along with pearls, small pearls, wisp wrappings, antlers and a few others that otherwise are never sold.

Two things of which to be aware (may have been fixed in patches):
1) If you revisit the Shrine before it's respawn timer has elapsed you can 'break' it and no more dremora will spawn there.
2) You have to buy something from Enthir as soon as you can otherwise he will not serve as a vendor after you become Arch Mage.

Other than those I found in the Nightcrawler Temple, I have purchased them at apothecaries in each of the cities or the orc strongholds. I have over 20 at level 52. Don't know what to use them for since my Bosmer wears fur and hide armors.

There is one in the companions hall that respawns too.
During the Azura quest, you have to fight a mage in the Azura star. He summons daedra. You get those hearts off the daedra BEFORE azura teleports u out
Random encounters(necromancers) will sometimes have daedra hearts
In oblivion there are so many daedra hearts that i simply dropped em all. Wished i could transfer them to skyrim.

I'm just going to say it. Sorry, but I can't help myself.

"Inside a Daedra, of course."

To actually be helpful though, everyone above is right. Shrine of Dagon, the College of Winterhold, and the Companions' Hall are all good places to find Daedra hearts. Also the alchemy shops.

That sounds messy - can't we just have them delivered?

After your char reaches a certain lvl you can buy them in the Alchemy shops, one of my char's has 6 so far just from the shop in Whiterun. $650 a pop. She just buy's them everytime she goes to town then stores them in a chest in Breezehome. Not like she can make anything with them yet, don't have enough smith skills!!! But by the time she has the skill's she will have enough hearts. ( for some dang reason I buy'um evertime I see'um even IF that character is still running around in fur's at lvl 30.)

I just purchased two from an alchemist in an orc stronghold. I did notice that after a certain level (30ish I think) I saw them more often in shops.

You can also turn a human heart into Daedra heart at the attornach forge, but human heart is even harder to find than Daedra Heart. Thoes mages must be missing something.