Pinterest is an easy option that has tons of options, otherwise I personally like to use Unplash just because they are royalty free stock photos so I can have high quality and not worry about stealing from someone else. However it is completely for personal use so it’s not a problem. Another option if you’re wanting vectored graphics as well as stock images is to use a site like Freepik which you do have to pay $10 a month for, but I get all of my vectorized graphics.