Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download “55 Skybox” pack Free.
55 Skybox pack v1.0
This pack includes 55 morning-day-evening (see screenshots below) skyboxes with different color and style variations (realistic-fantasy).
Each of the skyboxes contains 6 textures (2048 x 2048* in PNG format per each texture) and Unity 6-sided skybox material.
Usage: Just put one of 55 material into Windiw > Lighting > Skybox slot.
* IMPORTANT: All textures are imported at 512 x 512 for fast and correct import process in your Unity. You can adjust resolution up to 2048 in inspector for higher quality.