I'll have to go along with Brent and Michael. I bought a zenitar 16mm for my Canon 300D almost 3 years ago and I think it is great. At that time I paid $130 for it in a Canon mount. You can also get them in NIkon, Pentax (K), Pentax M42, and some other mounts.I got the Canon mount and wish now I had got the M42 mount as there are adaptors for a multitude of cameras. I would love to use it on my Canon F1 (film ) and now will have to buy a new one in the M42 mount. I know it has increased by now, but it is less than half the cost of Canon, Nikon, Sigma, et al and is a great lens. If you want fish eye effect, all you have to do is tilt the camera a bit, or be close to verticals, and it does a great job. I use it a lot for parties when I want to get the whole room in the photo and it really does a good job.
I think at current rate of exchange, you can get one for the Nikon for less than £150. As others said, check on ebay. you can do a search or do a search on google.
Hope this helps
Michael