I also found some maps for the moon and the sun. Now I also want to have the milky way in the scene. The closest I could find was this star-field which works fine, but does not feature the colorful milky way. After a few hours of fruitless searching I'm at the brink of creating my own using other sky-map sources, but thought I'd give it a shot to ask here before putting effort into this. I need a good high resolution source that can be mapped seamlessly on a sky-sphere in PovRay.
Skysphere is a slightly obscure term. Try this one:
There's various freebie apps around for converting between sky spheres and sky boxes if the skyboxes don't work for you.
Similarly you can "fake" a sky sphere or sky box from a good sized image with those same tools.