This is a paid asset, but now you can download Super Simple Fake Clouds for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Super Simple Fake Clouds v1.1
This asset is easy to use and gives you moving clouds for your sky without a mentionable impact on your performance.
Get and use these clouds if your sky is too empty or clouds in your skybox are too static. Quick and easy to use, drop one (or more) of the prefabs in your scene, adjust wind speed and rotation to your liking, maybe adjust height and scaling to fit your scene. Done.
Due to its simple approach and easy use this asset is perfect for your prototype, mobile game or VR experience. Needs no special shaders or hardware capabilities. If Unity builds for it, this works!
All sources included, modify as much as you want.
Feedback always welcome!