In the website ZBrushCentral you can download freely 24 HDRI create by the user abhaya.
 
All hdris are equirectangular maps 5000 x 2500 px and can be used in your private/edu and commercial projects, however you're not allowed to redistribute them on your website, blog, via torrent etc.
For complete information, in order to access these images, we need a completely free registration HERE