Where can I find an Ebony Helmet?

Solution 1:

Boss chests at the end of a dungeon are your best bet, since those things are random. Keep in mind that ebony armor will only start showing up after you reach level 32, and even then not frequently.

At higher levels, mercenaries (a random encounter) will have a chance to be equipped in ebony, apart from those ebony armor is very rare.

Solution 2:

You can get a FULL ebony but it doesn't give you the helmet if you do the easy quest Boethians Calling. The armour is enchanted with silent movement and enemies who get too close get automatically poisoned for 5 dps. Nice for such an easy quest, just take a follower and it's no bother.