Have a look at www.1000skies.com, they have some downloads, and you can purchase
very high resolution images.  They are generally 360 degree (spherical)
panoramic images for use in 3D rendering software, though you could just take a
cropped area.  Note that the lines of cloud are curved particularly in the
uppper half of the image.

Hope that helps.
Mark

Hmm... let me guess, you want to cut out some yukky grey overcast sky
from your house images and replace it some nice clouds? Would this
"free" photo be used for commercial purposes? ;)