The first thing you need to do is get a list of the clothing that is sold by the Radiant Remnant as they have every article of clothing in the game. Well, maybe not the enchanted ones, but the regular clothes.
http://www.uesp.net/wiki/Skyrim:Radiant_Raiment ~! Only noticeable thing missing is the Tavern Clothes, which is worn by lllladies like Alva in Morthal and Hroki in Morthal. Aside from murdering them/stealing it off them with high enough Pickpocket, it turns up in random wardrobes/dressers and on the occasional enemy or in a chest or something like that.
But yeah, there's also some special clothes that are either unique or extremely hard to come by. There's a set of Noble's Cloths (similar to what Jarls where) and some Fur-Lined boots in the East Empire Company Warehouse. Two unique clothing sets are obtained via the Dark Brotherhood questline.
Spoiler During "Bound Until Death," if you can escape without being noticed or want to run in without about two dozen guests and guards on your back, you can take the wedding gown and acessories off of Vittoria Vici.
In the last quest of the main line, "Hail SIthis!," you can take the Emperor's Robes off his body.
Honestly though I'm unsure if those can be enchanted since I've never tried to enchant them. Worth a shot though.
