I want to get a fish eyed lens for my Nikon, its a D3100?

I just don't know what kind i should get, or any details, or where to get it?

Here's a good place to do research.
http://www.bhphotovideo.com/...video.com/

You might like this one.42x HD Super Wide Angle Panoramic Macro Fisheye Lens For The Nikon D3100, D7000 Digital SLR Camera Which Have Any Of These (18-55mm, 55-200mm, 50mm) Nikon Lenses

INCLUDES 2 PRODUCTS -- with all Manufacturer-supplied Accessories and Full USA Warranties:
<#1> .42x HD Super Wide Angle Panoramic Macro Fisheye Lens + Lens Case, Caps
<#2> DB ROTH Micro Fiber Cleaning Cloth
* Please Note: Adapter Tube / Ring will be provided if needed

Save your money and only buy Nikon.

AF DX Fisheye-Nikkor 10.5mm f/2.8G ED