If your just in Tokyo then you should definitely it some of the second hand stores that stock Yohji.
DC Bank, Bring, Ragtag, Rinkan and Modscape all have locations in Shinjuku if I remember correctly. I'm not sure if the Ragtag and Modscape Shinjuku stores sell Yohji but worth a try.
Also with the Yohji Aoyama don't have high expectations. It's just another retail space; there's definitely more interesting stuff out there (infact go to 1ldk or pool aoyama if...