Rather than being purchased, the Chicken Zombie was given away to players right before the update to the game on May 17. If ...

as it's done from within the game. To do so, follow these instructions: Launch Zombie Uprising from the Roblox app. Open the chatbox, enter one of the codes below, and click 'enter' on your keyboard.

Roblox Grow A Garden has many things you can do, including apparently getting a chicken...who is also a zombie. Go figure.