Was looking for some (free) sky backgrounds for renderings and found Open Eye Visuals. Also got some free ones at 1000skies that are 2000px wide which are more than enough for my needs (higher resolutions and the full 360° skies can be ordered for a fee).
Edit: Open Eye Visuals, linked above, is no longer in existence.
Edit: Open Eye Visuals, linked above, is no longer in existence.