Extension: Insert icons for Docs
Extension: Insert icons for Slides
Website: IconArchive - Click the ICO in the upper right corner to download the icon.
Website: iconmonstr - these are black and white icons
Website: The Noun Project - this is a freemium site. Black and with icons for free version unless you pay.
Website: JoyPixels 6.6 - You have to create an account
Website: SlidesCarnival - offers slide templates for Google Slide and PowerPoint. On each template there are around 80+ icons you can use.