The only way to get the items is to own the Bioshock Infinite Season Pass on Steam, which will give them to you at no additional cost. If you already own a Season Pass, you should receive the items soon.
The items include a Little Sister's Big Daddy Pocket Doll, "sure to add some protection from a crit or two," and two masks (hats, basically), one of President Washington and one of Benjamin Franklin as they appear in Irrational's game.
The Bioshock Infinite Season Pass costs $20, and includes the combat focused challenge mode Clash in the Clouds, new items, and the two episodes of the Burial at Sea DLC.
The second episode of Burial at Sea, which Ken Levine said will be significantly longer than its predecessor, should release in late March.