Cloud's Kingdom Hearts uniform is dlc (downloadable content) and is not unlocked in game, but neither can it be bought. The only way to access it is to either have boughten birth by sleep final mix for the JAP version, or preordered the USA version. Maybe in the near future it will be on playstation network, but not today.
Source(s):
Siliconera
answers.yahoo.com/question/index?qid=20110326033316AAj1qXh