Creating great mobile apps usually involves making use of icons. May it be as touch target, indicator or just as decorations, icons make an app come to life and provide personality. Therefore it is important to focus on your icon design by and make them as readable and meaningful as possible.
A mobile platform usually has its own style of icons with certain key metrics and attributes. One of such icon systems is the Google material design icon set. It is the default icon set on Android but can also serve as a great basis for iOS and web based apps.
The material icon set provides an exhaustive list of downloads in a variety of formats: from pngs in various sizes (for iOS, Android and web), to svgs and webfonts. You can find a description on how to best consume the icons here.
Since the icons are fully open source under a CC-BY license, you can use them as a basis for your own custom icons. To stay within the realm of the material design system and not go astray, you should follow these icon design guidelines.
With all these great icons at hand, there is no excuse to have subpar icons in your app. :)