You can find high-quality , specifically optimized for MacBook Pro Retina displays, across several specialized wallpaper sites:
Sunset Over The Pier - Optimised for the Retina display - 2880 x 1800 2880x1800 Sunset HD Wallpaper and Background Im...
and Pexels are excellent sources for royalty-free, high-definition photography. You can browse their Sunset photo collections to find raw images that can be cropped or set to fill a 2880x1800 screen. You can find high-quality , specifically optimized for
offers a dedicated collection of Sunset Wallpapers at 2880x1800 resolution, featuring everything from minimalist after-sunset glows to fantasy-style sky reflections. You can find high-quality
hosts specific high-res pins like this Sunset Over The Pier which is explicitly optimized for 2880x1800 Retina displays.