Blue Skies…
cgskies.com boasts over 300 panoramic high dynamic range (HDR) sky scenes. Most of these are, naturally, daytime or sunset scenes, and have resolutions up to 15k pixels wide. I haven’t tried them out in our dome, but they look promising for use in fulldome!
Also check out their resources page for tutorials and links to software to convert cylindrical equirectangular panoramas to other projections.
Make sure to look over their terms of use before purchasing images for use in your productions. (Not that there’s anything there that should be a concern; it’s just always a good habit.)