Google has it
On Google image search you could specify which license you need. Quick and easy!
Flickr is one of the dinosaurs of images on web. Try a search! You may specify a license.
Unsplash is one of those sites with free images, that seems to be too good to be true. They are stating that all images are having a public domain license (CC0). However there is no way preventing people from stealing pictures and uploading them here. And you might get in to trouble as the editor.