Iconfinder
Friendly interface, clear navigation
URL：https://www.iconfinder.com
Google Official flat icon manual
Collected up to 10000+icons, including the vast majority of icons for web pages, layout, audio, business, and other themes. The formats include SVG, PNG, ICON, and font icons
URL：https://material.io/icons/
Icons8
It does not require downloading icons, simply drag the icons into PS or AI
URL：https://icons8.com
Iconfont
Rich set of vector icons, completely free of charge
URL: http://www.iconfont.cn
Fontawesome
Icon font library specifically designed for Twitter Bootstrap
URL: https://fontawesome.com