From your question, it sounds like you have a Sony camera, and that camera does not have a removable lens. If that's the case, you cannot use a Nikon fisheye lens with your camera. A fisheye lens from Nikon is designed to be the only lens on the camera!! It's not an add-on accessory that you can just slip on to the end of an existing lens, or anything like that. While lens converters can allow you to put a lens on a mount that it's not designed for, a lens converter goes between the base of a removable lens and the camera body -- not on top of the lens.
Based on what I understand of your goals, you might be better served by selling your Sony camera, and purchasing a new camera body with interchangeable lenses. The Micro Four-Thirds system should be of particular interest to you; the system's design goal is to provide affordable high-quality digital cameras that you can swap the lenses on, so a variety of lenses are available new and used from a variety of manufacturers.
Even new, you can find plenty of affordable fisheye lenses for the Micro Four-Thirds system like this 9mm/f8.0 for only $100, or, for a real bargain, this kit of three different lenses from Lomography including a fisheye for $90. These won't be good lenses, but with fisheyes you're really going more for special-effect than Pristine Optical Quality. Of course, if you really want quality, you can get this lovely Zeiss 15-30mm f/2.9 zoom lens for the low low price of $23,900 (!!!!! $$$$$) Micro four-thirds is a very flexible system. :P
(KEH.com has good listings of used camera bodies and camera lenses, with meaningful quality ratings, though it helps to know what you're looking for first because they don't do a good job of explaining their listings. If you are concerned about affordability, buying used lenses is a very good deal.)