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.

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.
A collection of customisable characters & scenes illustrations. Free vector illustrations for personal & commercial projects
Beautiful, free illustrations. Updated weekly. Hand-drawn vector illustration and icon resources, perfect for your next project.