Free, high quality, open-source icon library with over 1,300 icons. Include them anyway you like - SVGs, SVG sprite, or web fonts. Use them with or without Bootstrap in any project.
I imported the fantastic and simple icons from the Bootstrap project into a Sketch file using the Sketch Icons Plugin, and without any adulteration except making them a neutral grey. Add as a library for access to a huge amount of icons, probably the last icon set you'll need.