There’s lot of discussions happen regarding the use of copyright photos as references. As long as you are using a copyright image to practice your skills and the final drawing/painting does not look like a close copy of that image, there is no problem. You should not use copyright images if your goal is to sell it.
Below is a list of my favorite websites with copyright free images. However, the terms and conditions could be changed any time, so please be sure to double check.
Copy-Paste link into your browser if you can’t click on it.