This guide is split by area in the order you should try picking them up, you are free to pick them up in any order though assuming you can access the area.
If you're sure its the right place but you haven't got the item from it leave the area and return and you should find it.
Police Station
(1)-Dutch – in the vending machine in front of the room where you talk to Rachel.
(2)-Jagz – behind the police sign that says “police department safehouse” it is in a trash can before you actually go inside the police station.
Town Square
(3)-Chewy - this card is in the black car that is beside the shotgun ammo.
(4)-Mo – in a trash can inside the park itself (north side)
(5)-Gareth –to the far right of the level you will see a red vending machine you will find the card there
Suburbia
(6)-Russell – whenever you enter the area keep walking and there will a house to the left, there is a yellow bus, police car and a red car in front of it and beside the stone wall where the white gate is knocked down there is a crash can this card is in it.
(7)-Fats – right before you get to the end of the level to go on to the bridge go into the house on the right and in the clothes cabinet the card should be in there.
(8)-Lebowski – go past the yellow bus, police car and the red car and you will see a black car, search the trunk of the car and the card will be in there.
Bridge
(9)-Kingsley – from the starting area head north and you will see that the road has been blocked off look to the right and you will see a green box, the card is inside there.
(10)-Jody – from the starting area go across to your left then head downwards towards southwest, you will see a building (motel) inside the clothes cabinet the card will be inside there.
(11)-Xena – inside the same building as Jody, you will see a red vending machine and this card will be inside it.
Mall
(12)-Rich – as soon as you enter the mall you will see a red vending machine, search it and the card is in there.
(13)-Dave – from the entrance head south east and you come to a clothes store, check the filing cabinet that is in the store and the card should be in there.
Tv Station
(14)-Rob - from the starting area head north and you will see an opening into the tv station, there is a red vending machine search it and the card will be in there.
(15)-Jon - From where you found Aaron head down south until you exit the building. You will see a black car to the left, search it and the card will be inside it (the car is near the gold globe)
(16)-Aaron - In the same room where you find the guy (cant remember his name he is in the panic room) there will be a red vending machine to the right this card is inside it.
Power Plant
(17)-Tanguy -from the starting point of the level goto your right until you hit a dead end, then go up north a little you should see a red vending machine the card is in there.
(18)-Ed - in the room where you are introduced to the new zombies/where luxo is hiding in the panic room there is a vending machine to the right hand corner, this card will be in there.
(19)-Stuart - from the starting area walking straight into the "restricted area" which has all of the power generators, in the corner of the left hand side there should be a tool box. (The dev card will appear after you have clamed the power station as your base, the first time you open the box it may be a knife card or something like that.
Park
(20)-Kwok -From the starting point head south and you will see a green ammo case, the card is in there.
(21)-Gilby - from the starting gate head northeast, you will come across a trash can, the card is in there. (the crash can is beside the fairest wheel)
(22)-Jim - From the bumper cars, head south and you will see a small bridge. Cross over it and you will see a vending machine, this card is in there.
Credit to Sakura4600 from xba
and to GodLike499 with the name corrections.