The following costumes can be unlocked by spending the indicated number of Justice League of America Shields that can be collected throughout the game:

Batman (classic blue): 59 Shields
Batman (Batman Beyond infiltration suit): 74 Shields
Flash (Jay Garrick): 21 Shields
Flash (Walter West): 48 Shields
Green Lantern (retro): 23 Shields
Green Lantern (bold): 30 Shields
Martian Manhunter (modern): 29 Shields
Martian Manhunter (true): 15 Shields
Superman (black recovery suit): 32 Shields
Superman (Earth 2): 65 Shields
Superman (Kandor): 91 Shields
Wonder Woman (biker): 61 Shields
Wonder Woman (toga): 44 Shields
Zatanna (classic blue): 25 Shields
Zatanna (Identity Crisis): 38 Shields