Sierra Madre playing card

 is a playing card in the Fallout: New Vegas add-on Dead Money.

Characteristics
Sierra Madre cards are like any other playing cards found in the Mojave. There are 21 of these laying all over the resort & casino (3 inside the main building which is not immediately accessible, and 18 more in the surrounding areas), and as you cannot return to the sierra madre after completing the DLC, be sure to collect them all. There is also a related challenge called Dead Man's Hand, which requires you to find 5 specific cards. The full deck (54 cards) exists in the game's data files but the remaining 33 cards were not utilized. Their small size makes them even more elusive and easier to miss than skill books.

These cards ARE able to be used when playing caravan when you return to the Mojave, however they must first be placed into a container and picked up again. Doing so will cause the individual cards to disappear from your inventory and become a part of your caravan deck. There may also be other methods of accomplishing this.

Villa

 * Ace of Spades: In the southern part of the fountain where you first wake up and gather your team. (A Dead Man's Hand card)
 * 10 of Hearts: Some distance north of the Villa Police Station and west of the room containing the 5 of Hearts, on the interior corner of a walkway to the southeast of a wall fountain.
 * 5 of Hearts: Some distance east of the 10 of Hearts and northwest of the hologram which can be turned on with a nearby terminal, in a 2nd floor room above one containing a tripwire and swinging girder trap, under the south end of a couch against the east wall.
 * 4 of Spades: Some distance north of the entrance to the Residential District, inside a room with some metal shelves at the northeast corner of of a cloud filled courtyard with a dead tree, at the bottom of an open metal cabinet against the east wall.
 * Joker 2: Inside a 2nd floor room above a gift shop some distance north of the doors to Salida del Sol South and southeast of a wall fountain, partially covered by a bed in the northwest corner.
 * 8 of Spades (Police Station): In the northwest corner of the cell Dog is located in. (A Dead Man's Hand card)

Residential District

 * Ace of Diamonds: Around the corner southwest of the door to the Villa, past a vending machine and tree stump, under the south end of a bench.
 * 7 of Hearts: Inside a 2nd floor room with a tripwire and rigged shotgun, southeast of the doors to the Villa and east of the wall fountain and stairs seen when entering, on a wooden coffee table with two bottles of dirty water.
 * 2 of Spades: Inside a 2nd floor room with a pressure plate, rigged shotgun and fire extinguisher trap, with cloud covered courtyards to the south and (along with two dead trees and three tree stumps) to the east, behind a bed against the north wall.
 * 9 of Hearts: Inside a 2nd floor room southeast of Dean's place, behind a bed in the southwest corner.

Medical District

 * 9 of Diamonds: Inside a 2nd floor room just west of a speaker and some distance east of the Villa Clinic, under a small ruined book with a chessboard on a rectangular table next to an orange chair in the southeast corner.
 * King of Clubs: Inside a 2nd floor room with an active electric hot plate, north of the Villa Clinic and above a ground level cafe sign directly west across a courtyard from the room containing the 6 of Diamonds, on a round table in the northeast corner.
 * 6 of Diamonds: Inside a 2nd floor room northeast of the Villa Clinic and directly east across a courtyard from the room containing the King of Clubs, on a wooden shelf in the southwest corner behind an orange chair.
 * 7 of Clubs: Inside the gift shop northwest of the Villa Clinic's exit, on a chair next to a staircase and Sierra Madre poster directly in front of the doorway.

Puesta del Sol

 * 4 of Diamonds (South): Inside the same 2nd floor room as the Jack of Spades, north of Dean's position for the Gala Event, next to the small burned book on the top shelf on the northeast wall.
 * Jack of Spades (South): Inside the same 2nd floor room as the 4 of Diamonds, north of Dean's position for the Gala Event, behind a couch in the west corner.

Salida del Sol

 * Jack of Diamonds (North): Inside the northwest part of a display in the middle of the room with a hologram vendor, vending machine and bear trap.
 * 6 of Spades (North): In the northwest corner of a small courtyard with three bear traps and a dead tree, on the ground between a wall and three pipes, just west of the stairs leading to a room containing Dean's Secret Stash, a radio and two pool tables.

Cantina Madrid

 * 8 of Clubs: On the floor in front of a file cabinet to the right of the Cafe Madrid terminal and restaurant manager desk. (A Dead Man's Hand card)

The Tampico

 * Ace of Clubs: On the floor between a doorway and curtain in the southwest corner of the stage, on the opposite side of the wall from the ambient audio control terminal. (A Dead Man's Hand card)

Executive Suites

 * Queen of Clubs: On the floor in the southwest corner of Vera's bedroom, in front of her desk. (A Dead Man's Hand card)

Bugs

 * Relating to the Dead Man's Hand cards, it seems you have to carry them around the whole game, because placing them in a container and picking them up again glitches them into oblivion for most players. All of my cards turned into the 8 of Spades(Visually) after just dropping them on the floor, but highlighting the card shows another card. Dropping the cards on a table 1 by 1 (Preferably at your Safehouse) seems to be the only way to keep them without having them in your inventory and glitching them in a container. Be sure to save afterwards.
 * You can drop one of the Dead Man's Hand cards and pick it up again, keep on repeating this to complete Dead man's hand challenge
 * The 6 of Diamonds found on the wooden shelf in the Medical district is invisible, but still collectible.
 * Some cards, including the Jack of Diamonds, Jack of Spades, 10 of Hearts, 9 of Diamonds, 6 of Spades, 5 of Hearts, 4 of Spades, 2 of Spades & Joker 2, may be missing from their locations. In many cases this is a clipping issue and may be resolved using similar methods as those used to retrieve the Duck and Cover! from the Residential District. Joker 2, Jack of Spades and the 4 of Spades have been recovered in this fashion with some luck/trial and error.