Cheat Mode
Examine the closet in Leanne's room during or after chapter 7. While her first line of dialogue is on screen, enter the code. After entering the code, forward to the next line of text, and you should recieve your items:
[Down], [Down], [Up], [Up], [Left], [Right], [Left], [Right], [Y], [X] - 8-BIT GIRL shirt.
[Left], [Up], [Right], [Down], [RB], [RB], [LB], [LB], [L3] - Platform Logo shirt.
[Y], [Y], [Up], [Up], [X], [X], [Left], [Left], [LB], [RB] - Club Famitsu shirt.
[X], [Y], [LB], [LB], [RB], [RB], [L3], [Up], [Down] - Hirakou shirt.
[RT], [LT], [LB], [RB], [Y], [Y], [Y], [X], [X], [Up] - Gemaga shirt.
[R3], [R3], [R3], [Right], [Left], [Y], [X], [LT], [RT], [LB] - Politan Suit (available after earning the Reindeer Suit in Neverland).