This is a paid asset, but now you can download 5 Night Skyboxes Free.
Detail this asset from Unity Store: Original Link
5 Night Skyboxes v1.0 (Latest version)
Get five beautiful Nightt Skyboxes for your projects
This package contains the following 5 Skyboxes:
> LS SkyBox Night 01
> LS SkyBox Night 02
> LS SkyBox Night 03
> LS SkyBox Night 04
> LS SkyBox Night 05
Each skybox is composed of 6 textures, each texture is in psd format with dimensions of 1000 X 1000 pixels in a skybox / 6sided material.
I hope you enjoy these skyboxes.