Successfully complete the game to unlock infinite ammunition, infinite items, and additional character skins.
Argus Mercenary costume in Machi Akko level:
You will reach a hallway after you fall out of a vent. There are three rooms in this hallway that require the lockpick to enter. In the third room will be a closet, just like the other ones; however this closet cannot be opened yet. Proceed to the bar, then go on stage and use the microphone. Sam will say something dumb. After he is done, turn around and look behind. A keycard should be somewhere. This keycard opens the previously inaccessible closet. Make your way back to this room and use the keycard to open the closet; the costume is inside the closet.
Camouflage Sam costume in Lighthouse level:
Finish sniping the man on the roof, then before you rappel down off the platform, stay on the platform. In the high right corner is the costume.
Displace Mercenary costume in Display level:
Go to floor 3. To learn what door has the costume, look through the optic cable. Once you see the green package, enter the room; the keypad lock code to the door is "5800".
The Displace suit is in the room next to the room where you have your last chance to save; the code is "5800". There are a total of nine suits.
Masked Sam costume in Penthouse level:
Go to Zherkezhi's apartment and make your way to the first room on the top floor. Take out the guard and go to the long dark hallway at the top left of the room. There is a keypad lock to get in; the keypad code is "5698".
National Guard costume in Manhattan Streets level:
Go to the open area just before you enter the penthouse. Kill the two guards and climb the pipe at one of the ends of the area. From there, go left and jump down to the next platform. The costume should be somewhere around the corner.
Shadownet Agent costume in Kokubo Sosho level:
Once you go down to the elevator shaft Sam will consider the Descent to Hell. There will be lasers at the bottom. Once you are down there find a ladder to get back up. Once you climb it, you will end up in a small room with lasers. The costume is somewhere in the back corner.
Short sleeved Sam costume in Bank level:
When you must steal millions of dollars by using the lockpick, go to the small vault left of the computer. Use the lockpick, and the costume will fall out.
Snow Guard costume in Battery level:
Find the wrench in the bathroom. The costume is in the room that is shaped like a "U", and has two cameras and four lasers. There is a keycard room in there. Once you enter the keycard room, in the right corner is a guard standing in the dark. Take him out and climb on the crate. Then, turn towards the next crate and select the option "Bash Door"; the costume will be in there.
Thermal Suit costume in Hokkaido level:
Before you try to find the costume, locate the wrench that is laying on the ground. Then, proceed through the level until you reach the man who is shooting from the top of the building. Once you kill him, get to where he was. Then, run across the rooftop to the vent. Exit the room you fall into. If done correctly, the icon "Bash Door" should appear in the top left corner of the room. Bash the door and the costume will be in there.