For more detail this asset from the Unity Asset Store: Click Here
This is a paid asset, but now you can download Fantasy Skybox 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.
Fantasy Skybox v1.6.0 (Latest version)
Fantasy Skybox v1.2.3
“If possible, please buy the package to support the developer”
Fantasy Skybox contains 34 high-quality skybox. There are hand-painted textures for sun/sunless and moon/moonless. Demo scenes let you navigate through all skyboxes to find which one is the best for your game.
Demo on YouTube
How to use demo scenes
Features:
• High-quiality hand-painted textures.
• Contains 34 skyboxes.
• Contains Sun and Sunless skyboxes.
• Contains Moon and Moonless skyboxes.
• Contains demo scene and sample scripts.
• Supports all build player platforms.
Compatible:
• Unity 2017.3.1 or higher.