At the starting screen main menu (Your Character's Bedroom) enter the Konami Code (Up, Up, Down, Down, Left, Right, Left, Right, Square, Triangle, Start) to get a free Robbie Doll weapon as well as enable the sale of Robbie Dolls at the shop. You'll know the code worked if you hear a child's laughter. You need to input the code each time you load up your character. If you reload your character, you can re-use the Konami Code to continuously get more free Robbie Dolls.