This is a paid asset, but now you can download Skybox Green Nebula Free.
Detail this asset from Unity Store: Original Link
Skybox Green Nebula v1.0
“If possible, please buy the package to support the developer”
Skybox Green Nebula is a high quality space skybox made up of six 2k textures.
• 2048x2048px Seamless Textures (resizable)
• Includes sample scene
Asset is also included in Nebula Skybox Pack
Unity 2019(+) compatible
Support available for assistance if any problems should occur.