I think I find a new free illustration site about onceamonth, and here’s another one. DrawKit offers a bunch of illustrations for free, just scroll to the bottom of the page.
The illustrations offered by DrawKit are in .png and .svg format. PNGs are graphic images you can use right away, while SVG is a vector drawing format. Since SVG is easily editable in an app such as Inkscape or Gravit Designer, you can take one of their illustrations and modify it for your use.