Over a million icons are available in the Noun Project.
Need an icon for that presentation, project, or website? Then the Noun Project is for you. Each icon can be downloaded as a PNG or SVG. Some icons are in the public domain and can be used as you wish, while others require attribution or have some other license requirement.
Educators can get royalty free access for $19.99 a year by upgrading to Noun Pro. This includes access to all of the icons through the website or various programs and add-ons. One such add-on exists for Google Slides. Without Noun Pro you only have access to 100 of the most popular icons with the add-on.