Twilight Render User Forum

Free and seems like good quality spherical skies. Have not tested them yet though.

http://blenderartists.org/forum/showthread.php?t=24038

Thanks Sepo,
I recognize those maps - I believe they are linked from Kerkythea's forum as well.
They were not very "high"-res when I used them. They do appear to be higher res. from the download you link above.
The rectangular versions say "spherical maps" but they are only Hemispherical maps... so they need to be loaded in Twilight in the Hemispherical Sky slot.

Are these skies computer generated? The clouds in some look unnatural.

I believe many of there were computer generated, and some were photos, or photo-montaged with computer generated skies.
They are not very professionally done from what I've seen. They are ok for "starter" kit.