Ha I came across this question in another forum and thought i'd share...also like to know as well:

"Is it possible to get an executioners hood in Skyrim? I want to put it on with my Nightingale robes to look like the [censored] Batman."

Yes. First quest after killing the Old Woman in the Orphanage requires you to do something that leads to a lootable body with an executioners hood on it.

Yes, yes it is. I have one. IIRC, I got it from the first quest in the DB...there are up to three you can obtain there.

1. Complets the quest Innocents Lost - http://elderscrolls.wikia.com/wiki/Innocence_Lost
2. After the Innocence Lost quest has been completed, a courier will appear and give you a note with the imprint of a black hand and the words "We Know" written beneath it.
3. Sleep in any bed and the quest With Friends Like these will start - http://elderscrolls.wikia.com/wiki/With_Friends_Like_These...

Then follow what the Dark Brotherhood member tells you todo and search the body and there is an Execution mask for each person you kill.

Execution mask is not an Executioner's hood anyway. It has no eye sockets, it's just a black canvas bag you put on your head.