I come across this HQ wallpaper site...
http://www.ehdwalls.com/
As far as I can see, it's probably all stuff found on the Internet over the years, so giving due credit may be an issue (unless you don't mind crediting anonymous Internet find). The collection is split into different resolutions and subjects (use drop-down box top right and you'll see what I mean).
The "Space" sections have some great shots that would work for render backgrounds - real shots (probably NASA ones), digital renders, and real-digital combos too.
