WallpapersWide. Think of WallpapersWide as the grab bag of Retina wallpaper sites, offering everything from cartoons and celebrities to “motors,” music, nature and “vintage” backgrounds (you can sort by any of those categories, and dozens more).
The only caveat: Many of its wallpapers top out below resolutions like 2560 x 1600 or 2880 x 1880, so be sure to use the handy “Filter By” resolution option on the left column. (That said, sorting by 2880 x 1880 turned up well over 400 pages of material, with 10 pictures per page.)
Cost: free