You can find zoo decorations in the decorations section of the Store in the tab marked with a paw print. These decorations can be placed both in the Zoo and in town once you have a certain number of animals living in your Zoo. If a decoration is unavailable, you'll see the number of animals still needed to unlock it at the bottom of the decoration card. Zoo decorations don't have a practical purpose, but they can significantly brighten up your town and Zoo scenery.