Note that you should have a corresponding card for each of the Disk Crates found throughout the Game. I don't remember all the names, but this does go in order of each one you can find.

Some cards will unlock Crates with Health Bonuses, but most will grant Developer Commentary.

Many cards are behind hidden hallways/doors. These are enormous and should be easy to check
with a quick glance from the Scanner. Use the landmarks in my screenshots to get your LoS right.


First, there's a card next to the Antique Lighter.


Second, a card is hidden on a suspiciously empty ledge in the Catacombs.


Third, in the Meat Grinder area, a card is next to one of Mr. Tops' viewer screens.


Fourth, after getting the Revolver, a card is hidden in the hallway behind the Monster's spawn area.


Fifth, a card is placed behind the Eagle statue near the Funhouse ride entrance.


Sixth, during your first encounter with buzzsaw-barricades, a card is near the trap's switch.


Seventh, in your second encounter with the buzzsaws, across from the hidden Altar room.


Eighth, a card is in the Sawtooth's workshop area.


Ninth, a card is in the pile of guts under one of the first Mallet encounters, near a switch.


Tenth, another card is placed in Bam Bam's workshop area.


Eleventh, a card is hidden near your first encounter with the Floor Traps.


Twelfth, during your encounter with the rotating Sun Mouths, a card is in a hidden hallway.


Thirteenth, a card is hidden in the Connector puzzle area with the yellow neon paint.


Fourteenth, a card is in the Green Leviathan corridor to the left of the Purple Tentacle lever.


Fifteenth, a card is placed in the central hall of the Yellow Leviathan corridor next to the two gates.


When you kill Mr. Tops, be sure to use the cards to activate the Developer's gate near the Exit.


Talk to the Developer and keep shooting the Source Code.