Wannathis - 3d illustrations and freebies
Humaaans - Mix-&-match illustrations of humans with a design library.
Shapefest - A massive library of 3D shapes for everything!
Control Illustrations - 108 free flat illustrations with customizable characters
Iconscout - Design Resource Marketplace.
Ouch! Illustrations by Icons8 - Professional, perfectly matching, and customizable illustrations for any designs