There are several free stock photo repositories such as Unsplash, Pexels, and Pixabay. You can also try filtering Google image searches for photos with a creative commons license.
If you’re willing to pay, high-quality images can be sourced from repositories like Adobe Stock Photo, Shutterstock, or iStock.
If you have an HD camera on your smartphone or DSLR, you can try taking photos yourself. You can also create graphical images on your own using design tools such as Canva.