During the thieves guild youre given the option to upgrade a piece of the armor they have given you, this is a miscellaneous topic and is supposedly "optional". If you talk to the npc you go to to do this and dont have any of the armor, she says youre out of luck. Doesnt seem like a big deal.
Later when trying to finish the thieves guild questline by becoming the leader, you are told to return to the same npc to retrieve your guild leader armor. But if you have not upgraded one of your original pieces or armor that is all the npc will talk about. There is no way to get your guild leader armor. She has no other speech options, and you cannot get back your original armor if whoever you sold it to has rid it from their inventory. There is no spare armor laying around in the guild and you cannot pickpocket off another member or even kill them and take it, its not in their inventory.
YOU CANNOT FINISH THE GUILD IF YOU SELL YOUR THIEVES GUILD ARMOR
So Dear Bethesda,
Please fix the fact that you CANNOT get your guild leader armor and finish the thieves guild questline if you sell your armor so you dont have a piece to upgrade. Make tonalia or whatever have another speech option even if you didnt do that or make a way to get more thieves guild armor, because its rediculous that you cant finish the questline because you sold some crappy armor.
Regards, Joshimodo

Read my post here: http://www.gamesas.com/index.php?/topic/1295341-cant-finish-thieves-guild/page__view__findpost__p__19515366

I am helping those that send me a save file get the thieves armor.