This is a paid asset, but now you can download Toon Skyboxes Free.
Detail this asset from Unity Store: Original Link
Toon Skyboxes v1.0
Toon Skyboxes perfect for any stylized background.Fill up the backdrop and add mood to your game with collection of 10 Skyboxes.
-4 Day Skyboxes
-2 evening Skyboxes
-4 Night Skyboxes
We will be adding more to this collection with time.