Some new downloadable ones from MS here
http://windows.microsoft.com/en-US/windows/downloads/personalize
http://windows.microsoft.com/en-US/windows/downloads/personalize
my tip: download all the .themepack files into a single folder. highlight them all and then extract the contents (using 7-zip or other archiver) into another folder. now you have all the wallpapers without having to use the cumbersome themes interface.