Icrontic have an interesting article for the DIYers out there:
The high price of a fisheye lens often discourages a lot of potential buyers. A quick glance online will show that even cheap Russian lenses with conversion mounts are $300 or more. For amateurs, the fisheye lens is usually nothing more than an interesting gimmick; if that's the case, the price seems difficult to justify.
In lieu of a fisheye that clears out the bank account, bide your time with one you can build with just $10 and a thirty minute trip to the local hardware store. All you'll essentially need is a combination of PVC pipe, electrical tape, and a door viewer or peephole for this task.
Read the full article at Icrontic:
The fisheye made in this article is intended to fit onto my Nikon D50’s f/3.5-5.6 18-55mm lens. This is the stock kit lens for most of Nikon’s D-series. The same methods here will work on any camera lens, but requires a bit of flexibility considering the sizes of many lenses vary. I suggest you bring your camera with you to the store to find the best fit possible.
Manual focus for the camera will work best. The first thing you may notice when you take photos is that the fisheye is not full format, but the ring actually can have some interesting effects. Since the PVC pipe is white, light can bounce around on the inside and change the color of the exterior. If you are averse to the border, you can use a lens with a greater zoom and it will reduce or eliminate the circle.