Alas, my KH cosplay is a WIP at the moment. But I do have the solution.... ish
What you could do is buy a leather jacket (cheep one from the Op shop) and just buy a meter or 2 of Pleather or Vinyl from a local spotlight and just add onto it! (since it is just a costume party) make gloves and you have a Organisation XIII coat! you could just add a pink cheep wig and call yourself Marluxia
Hope this helps!
*insert Witty Signature here*