Public Domain and Royalty-Free Graphic Images
This site has over 130,000 pieces of royalty-free clipart. All clipart is in the public domain and thus available for commercial use.
Pixabay has over 1.3 million images and videos. All contents are released under Creative Commons CC0, which makes them safe to use without asking for permission or giving credit to the artist – even for commercial purposes.
This site lets you search for icons, however many of them are not royalty-free. If you set the search parameters correctly you might have better luck.