Freepik Incredible vector library from Spain. It has photos too. Some materials are not free.
Gratisography Stunning copyright-free images. Many are in high-resolution.
lukaszadam.com/illustrations Some beautiful, totally free illustrations that you can modify freely as much as you want.
OpenPeeps Superb collection of hand-drawn illustrations in Figma, Sketch, and XD formats. Free.
UnDraw.co/illustrations Lots of illustrations. SVG & PNG.
Humaaans Mix-&-match human figures and create the human figure that you want.
DrawKit Some nice free illustrations. It also has a newsletter with free assets.
joeschmoe.io Illustrated avatar collection with API.
StockUnlimited Huge vector library. It’s not free, but you can sometimes find cheap lifetime deals if you search Google.