One of the things I found is a Kenko Fish-eye 180° adapter.
You mount it onto a lens and you have a fish-eye, the only thing you have to do is finding the right focal length.
The lens I used is a AF nikkor 35 -70mm 1:3.3-4.5 (old analogue lens) because I needed a 52mm filter thread to mount the fish-eye.
The right focal length to get a full circle was around 45mm. I had to add some extra vignette, because the original vignette of the lens was not ideal (it had some purple colouring in it).
Nikon D5000 + AF nikkor 35 -70mm 1:3.3-4.5 (analogue)
aperture priority , ISO400, 45mm, f /3.5, 1/100sec Kenko settings: focal lenght to use 50mm, f /8.0
Exposure: +0.4, Contrast: +10, Shadows: -17, Blacks: -5, Clarity: +28
And a little set-up photo