What mm fisheye? Nikon d50 camera?

Okay, so this is kinda confusing but… I'm kinda new at photography but I really want a fisheye lens. I have a nikon D50 camera, and i'm not really sure what MM my fisheye should be. I don't know if this helps but I want the pictures to look like this: http://www.photopoly.net/20-excellent-fisheye-photography/ i'm not sure if the MM affects the price, but since i'm really new i'd like to try not to spend a whole lot.

You can't get that look without the proper lenses and they are very expensive. The 10.5mm Nikon is $775 and comes close to that look. Those are shot on wider, more expensive lenses like the 8mm. I saw one on Ebay for $2, 499.00