For more detail this asset from the Unity Asset Store: Click Here
This is a paid asset, but now you can download “55 Skybox” pack for FREE, Please keep in mind this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
55 Skybox pack v1.0
“If possible, please buy the package to support the developer”
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 PNGĀ 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.