Where do I find backgrounds like this?

I saw this shot uploaded to Dribbble some weeks ago. And I really like the use of the backgrounds. Does anyone know where I can find backgrounds like it? I want ones with something like mountains or maybe buildings in bottom of the image, and then the rest is like sky or a blurred image. Just ones that's really good for web- and graphic design use.

Shot:

http://ift.tt/1GGrJK1

Thanks.