This outfit is unlocked after your initial play through, and can be purchased for $10,000,000 from the Gift Shop.
Money Grinding Method:
The easiest method (that I've found thus far) of accumulating money quickly is to set the game to Nightmare Mode and replay Episode 1. The Episode is 35-38 seconds long and will reward you with anywhere from $720,000-$729,000 money per play through on Nightmare Mode. REMEMBER TO UNEQUIP THE DARK BOOSTER AND ULTI-MONDO COSTUMES WHEN MONEY GRINDING.