UI8 is a marketplace for a variety of design-related assets, including illustrations.
I wanted to mention it here since it includes countless great packs. In most cases, when using illustrations in real projects, you’ll have to find one great pack and stick with it. So for example, when designing an automotive-related app, you might want to use some automotive-related illustrations. And these usually come in their own, dedicated packs. UI8 is where I look for those packs.