[Source] CG Skies
CGSkies.com is your source for high resolution panoramic HDR skies with over 300 fully panoramic skies with super high resolutions of up to 15.000 pixels wide.
These panoramic skies can be used as a background, source of lighting and reflection for your 3D Scene.
Each sky comes with example render, 3D preview and dynamic range preview so you can easily determine which sky will work for your project. Free samples of both the 8 bit and HDR version are available, so you can always test out the sky before you purchase.