When making a game, the best approach is, of course, to create a set of original assets made exclusively for your game. However, this might not be possible for different reasons, such as budget. Fortunately, there are plenty of free assets available online for game developers to use.
Use free assets to create a proof of concept of your game idea before you start investing money in custom-made assets!
You can find a sample list of completely free assets at http://android.kul.is/p/list-of-free-resources.html.