First off, Welcome to Cosplay.com.
But straight to the business. You probably won't find someone selling the costume for a while, as the game has only just released. Your best bet is to check ebay later, but I can assure you the quality won't be that well, and I highly doubt you'll be able to find one that comes with the sword and gun, and can almost promise they won't come with a devil bringer arm.
But good luck anyways.
"I enjoy eating pennies minted on the year of my birth."