I don't know about the costume, but my friend Dread Pirate commissions Key Blades. You should message him and see if he'll make you one. I'm not sure what the price would be, though. He's made a few Key Blades already; 4 of them have been at KCon the last 2 years.
And if you post pictures of the outfit, you might have a better chance at getting someone to commission it for you (especially those that aren't familiar with Kingdom Hearts that well, like myself)