Following are some free illustration libraries, which include character illustrations, isometric illustrations, background illustrations etc. which can be used in your projects or can be used as an inspiration.

Undraw
Open-source illustrations for any idea you can imagine and create. A constantly updated design project with beautiful SVG images that you can use completely free and without attribution.
Illustrationkit
A collection of customisable characters & scenes illustrations. Free vector illustrations for personal & commercial projects
Drawkit
Beautiful, free illustrations. Updated weekly. Hand-drawn vector illustration and icon resources, perfect for your next project.