A new Blender 3D tutorial on how to create yyour on HDR map based on a 3D environement.
This is not 100% accurate as you may face some subtle distortion on both pole but the result worth it.
EDIT :
A new vidéo is to com thanks to Tim Bahrij whom suggested a way better, more accurate and simpler way to create your owne HDR.
Here is the new vidéo, way more efficient and accurate.