Since creating games can become quite expensive, we'll use some of the best free assets out there. There are plenty of resources available to game developers, either as placement assets for the developer's use, whether they are full assets, or just a framework that you can tweak to get your desired result. There are a multitude of options.
Note
In the code bundle of this book, you'll get all the assets you need to follow during the creation of the game. The site where it is available online will be listed with the instructions.
Some of the best sites to gather assets are described as follows:
Art: Art, especially 2D art, is generally easy to find on a budget, particularly for the placeholder art, until you buy or create your own for the finished product (although I've seen many games created with some of these assets). Some good sites to start with are http://opengameart.org/ and http://open.commonly.cc/.
Audio: Sound that works for your project is a lot trickier to get. Free sites are okay, but they generally don't have the right sound you will want or you will end up digging through hundreds or more sounds to get a close match. A good website to start with is http://soundbible.com/.
General: Some sites just hold a general collection of assets instead of specializing in specific areas. The best site for this, as everything is almost guaranteed to be free, is http://search.creativecommons.org/. The Unity Asset Store also offers a great deal of free assets from art to code. Some of these assets are available from users, and others are available from the Unity team themselves. Not all of the assets are free, but you can easily sort your search result by price. You can find the Unity Asset Store at https://www.assetstore.unity3d.com.