https://material.io/icons/
图标素材,Access over 900 material system icons, available in a variety of sizes and densities, and as a web font.,
http://simplelineicons.com
https://fontawesome.com/icons
http://www.iconfont.cn
https://www.iconfinder.com
http://www.easyicon.net
http://www.iconarchive.com
http://simplelineicons.com