I am looking for the best light armor helmet or cloak, regardless the level i am at.

Any suggestions where to find unique items?

I use the leather armour that you recieve from the dark brotherhood or i use the black robe because it ways nothing and leather or fur boots because of the lightness of them. This combo with a good bow and some enchanted arrows helps keep enemies at a safe distance if you miss on the sneak attack you can always run faster with lighter stuff to get a better vantage point.

where do i get the black robe? is it just a robe for the head (cloak) or is it a overall robe?

it covers upper and lower body, so you can still wear boots and gauntlets and a hood. i think brother pinter at weynon priory wears one, if i recall corectly, but you would have to kill him to get it.

No, you don't. Just follow the main quest for a while, and you'll find him dead.

Edit: (It's actually Prior Maborel who wears that robe.)

I'd like to have a nice hood or helmet. No further suggestions?

BTW what effects does the black robe have?

Black robe has no effects, unless enchanted. If you join the mages guild and get access to Arcane University, you get to the enchanting altar or something like that (a sigil stone works too, anywhere).

Mage's Hood. Is zero weight. (a nice blue)
Aqua Silk Hood. Ugly and aqua.
Red Silk Hood. Kinda nice.
Mythic Dawn Hood. Nice shade of dark red.
Necromancer's Hood. Deep black.

Regardless of level:

Glass Helmet for Light Armor
Daedric Helmet for Heavy

With the Shivering Isles installed:

Perfect Amber for Light, Perfect Madness for Heavy

Hoods are nice, as well as street clothes, enchanted with sigil stones or your own enchantments, as you can have 100% spell effectiveness. Plus they look cooler than helmets. Except the Perfect Madness.

i could only remember pinter, but indeed it was maborel, and yes he dies during the MQ, steal the black hand robe and hood from j'ghasta (spelling?) in Bruma. it has some enchantments and they look good. they do cover up your whole body.

The advantage is that it's zero weight. You can enchant it with something like a shield effect, and reverse-pickpocket it onto an ally, and improve their chances of surviving.

There are several other zero-weight clothing items in the game, including the wrist irons you're wearing in the beginning, the dark shirt worn by Agarmir in the Unfriendly Competition quest, mages' hoods, and the rare Black Wide Pants.

Outside the walls of the imperial city is a shady guy (forgotten his name now) but he wears leather boots, black wide pants, dark shirt and has a black hood. Its important to know aswell that he sells potions ans skooma and poisons.

Shady Sam.

There's a necromancer named Jalbert in the Vilverin ruin. He wears the dark shirt and wide pants, and he also has some loot. You can pick up an "ancestor" statue while you're there, and sell it to start the "Collector" quest.

Go vampire hunting. Some of them carry/ wear black robes and it's only robes, no hood with it.

I am looking just for a hood or light armor helmet, not a complete robe. Any suggestions?

Best unique light armor helm = Fingleam. Google it for more info.

There are no premade hoods with stats in Vanilla (Edit: heh, forgot about Nocturnal's Cowl, see below). The Mehrunes Razor dlc has a nice red hood from the boss with some defensive enchants on it.

For cloaks, with or without hoods, you'll need a mod that supplies these.

If you want clothing, one of the magic shops (Rindir's Staffs?) sells some of the zero weight hoods and other zero weight clothing, although not the black hood.

Another alterantive is to join the DB. You get nice light armor: body suit and hood with enchants which is non-levelled. And of course there's my favorite hood. Nocturnal's Cowl, but you gotta finsih the thieves' guild line to get that.

Mara

Also from Mehrunes Razor, if you kill the Morag Tong assassin in the jail, you can loot his very nice (leveled) light armor, which includes a Black Hood with a couple of handy enchants on it (Detect Life, and I think fortify Marks and something else).

Unfortunately I don't have much more to add then what has already been suggested by others. I looked at the UESP Wiki, but it's hard trying to find a hood or helmet that is not leveled. I might second Gabril's suggestion about going to Bruma and stealing yourself some enchanted Black Hand Robes & Hood. The hood has some pretty good skill fortification enchantments, but it's an article of clothing and not armor so will not provide much/ any protection otherwise.