http://www.nikonchat.com/10-5-nikkor-fisheye/

I need a fisheye lens for my D3100!?
Okay. I searched and searched and searched. All I want is a fish-eye lens of good or adapter. I have a Nikon D3100. I already know that Nikon Nikkor lens has a 10.5 … But it's like $ 750. I want something under $ 500. An adapter is fine, but I want to be nice quality and the FA. Please help me out. I'll give you points. (What that means) lol. Thanks in advance!
I do not think you get the AF to a new fisheye. You can get the 7mm Fisheye Vivitar around $ 330, but it's a manual focus lens. But remember, a fisheye lens has a wide viewing angle, depth of field is very large, so it will be quite easy to get a picture in focus with MF.
Nikon 10.5mm f/2.8G ED AF DX Fisheye Nikkor Lens