I always use pics4learning in the classroom - but don't think you are allowed to sell them. https://search.creativecommons.org/
does a good job of filtering results for commercial purposes - but I would still check them carefully before selling anything. https://commons.wikimedia.org/wiki/Main_Page
is another one I go to.
I'm a bit of a cheapskate - I only use free things, but that can get a bit limiting if you are searching for very specific things. So long as you know a bit about creative commons licences then there is heaps out there.