I want to have a dark brotherhood costume that isn't enchanted so I can enchant it myself. Does anyone know where I can purchase/steal/find one?

www.skyrimnexus.com

Actually... I'm pretty sure I have found a full set of unenchanted Shrouded armor on my way through the Dark Brotherhood questline. You're good

Do you remember where?

I'm almost certain it was either while chasing Cicero through the Dawnstar sanctuary, or during the quest given to me after having my future read by the old woman in Whiterun. Almost certain.

Uhmm, no there isn't. Just like there isn't any thieves guild armor you can purchase. I hear ya and know where you are coming from. It would be nice to have the cooler "light armor" options and enchanted with what fits my play style. But, don't hold your breath.

theres stuff lying around the sanctuaries called worn shrouded armor i think. pretty sure they dont have enchantments.

It's called Worn Shrouded. You can find it in the Dawnstar Sanctuary.

The gloves appears to have the sneak attack enchant, but that's a bug. You can enchant them however you like.

yes there is a unenchanted set , i think it was on a bookshelf in dawnstar santuary it was during the quest where you have to kill cicero ithink the name of the set was worn shrouded armour etc

Its not proper shrouded armor, its like weakened shrouded armor, and its unenchanted.