Are you looking for an Tiff or JPG type image to use in a digital image program for your green screen effect?
Hubblesite.org has great images of space, but I don't know what the license for use is.
You'd need to try a stock photo place and buy an image for comercial use.
I'd create something in Photoshop:
Create some noise invert the color. Now you have Stars.
Add a layer or two with some clouds effect.
Add lense flare for extra effect.