Scout: Under the deck at BLU spawn
Soldier: On the Manhattan Used Fireworks sign
Pyro: On the crates in the credits room
Demoman: Behind the building on RED high ground
Heavy: In the toilet in the apartment
Engineer: Below the starting platform in the moving platforms room
Medic: Near the AC unit on top of the Cluckin' Bell
Sniper: Under a front row beach chair on RED's side.
Spy: In Big Konahi on or under a sofa. (Thanks to Pudding Gay)

Getting all of these ducks will open a gate to the skybox, if I remember correctly.