Below you can see the skyboxes panoramic view and also have a look around in the unity app.

When you download a skybox you will get the DDS file as well as the 6 Original images.
So you can use them in unity3D.

Snowy Skymap for sale $2

Nebula Skymap for sale $2

Red Planet Skymap for sale $2

Sand Stone for sale $2

Lava and Mist for sale $2

Lava and Clouds for sale $2

Car Garage ( As used in Avatar Driving ) for sale $2

Green Hills ( As used in Avatar Driving ) for sale $2

Night Sky ( As used in Avatar Driving ) for sale $2

Star field with the sun and moon for sale $2

Asteroid field for sale $2

Sea and Clouds for sale $2

Sky and Water Skymap for sale $2

Unity 3d Example of the skyboxes. (Drag with the left mouse button)

Sky box collection