You don't have any option besides looking for generic clothing and enchanting them yourself. This is the most effective way to get the best possible equipment for you.
Refer to the clothing list for choice items. Unlike Morrowind, every item has the same enchant value (I.e. you can't put better enchantments on an archmage robe than on an apprentice robe), so decide by item style. For info on how enchanting works, check the uesp wiki.
However, I wouldn't say the reduced spell effectiveness is a big factor, you only lose 5% effectiveness once you reach 50 in the armor skills and armor generally can hold stronger enchantments than clothing and protects you much better (you pretty much have to spend all your enchants on armor when wearing clothing or have to constantly keep a powerful shield spell up). Only go for clothing, if you want to do it for roleplaying or challenge reasons. It is much more optimal to wear armor.