Games require art assets such as textures, sound effects and music. There are numerous sites available that provide assets either free or at nominal cost.
Be sure to read the licenses on all assets you use. Sometimes all that is required is that you provide a public acknowledgement of the source's asset. Sometimes payment is required. Your game submission must include detailed acknowledgements of any third-party assets you use.
If you have favourite sites for assets other than those listed below, let me know and I will add them to this page.
The Unity Asset Store provides an amazing trove of sprites, models, textures, skyboxes, and other assets. Some assets are free, some are available at a nominal cost, and others are more expensive.
You are welcome to use art assets from the Asset Store, as long as you clearly acknowledge the source of what you are using. Any costs you incur are of course your own responsibility. There is no requirement to use professional art assets in this course - some of the best projects in the past have been based completely on art created by the student team.
When creating 2D art assets, you will need to create textures for use in Unity:
There are numerous repositories with high-quality textures available free or at low cost:
Games involve sound effects ranging over, for example, footsteps, creaky doors, waterfalls and gunshots.
If you wish to edit your own sound effects, consider:
Soundtracks can help establish the mood of the game. The best game soundtracks have no vocals. There are numerous repositories that offer music suitable for use in games.