I found something pretty awesome on 512 pixels. 512 Pixels posted each default macOS wallpaper from 10.0-10.13 and each wallpaper is 5k so it will look stunning on any Mac. Currently rocking the leopard wallpaper on my 2016 MacBook Pro 15" space grey 2.6 ghz.
link: https://512pixels.net/projects/default-mac-wallpapers-in-5k/
link: https://512pixels.net/projects/default-mac-wallpapers-in-5k/
