UI Faces aggregates various free avatar placeholder sources that you can use in your design mockups by copy—paste, API, plugins for Sketch, Adobe XD and Figma.

Avataaars is a free Sketch Library that allows you to quickly create avatar illustrations. Combine clothes, hair, emotions, accessories, and colors.

Avataaars Generator is a simple web app for generating avatars

Mix & Match Robots with a Sketch library Create robot illustrations in Sketch App with this free library. Combine frames, antennas, sensors, accessories, and colors. 🤖 Free for commercial and personal use.