More than 20 (currently 23) free, royalty-free and free for commercial use, high quality and high resolution (10,000 x 5,000 pixel) HDRI maps are available for download at HDRMAPS.

In fact, it's worth exploring the whole site as the price for the "pay content" is also affordable and the image resolution of the commercial hdri's is the double of the "freebies"; 20,000 x 10,000 pixel. There are bundles of hdri skies, too.

Some "samples" from the site...