If you dont find anything on Fur Affinity, there are also places like Deviantart, or you can do a simple google search... Myself i usually use the default windows backgroud if im to lazy to change it... or i use something i've drawn myself since i usually make my art in high resolution (usually 1000x1000, but it's easy to make into a background since i have the raw files and never delete the photoshop files)
Thats pretty much the only places i know though... byt my suggestion would be to just browse through the web a little til' you find something u like