Purchasing specialty lenses for your camera can be costly, especially when they're really only good for one style of photo. Stick to your traditional lens and utilise a peep-hole to create a fish-eye effect without the extra cost.
Adding things to the lens of your camera can be tricky business (no one wants to worry about scratching existing gear). Instructables user Banjomaster kept this in mind when creating his own modification for his existing camera set up. By using the lens shield to attach his creation to, it makes for easy removal without harming the lens or camera body.
The supplies include a fish-eye door viewer, a few scrap pieces of wood, a bit of duct tape and the lens shield. If you no longer have a lens shield or your camera model didn't come with one, most camera shops will carry them for a few bucks, or there's always good ol' eBay.
Hit up Instructables for the full tutorial on taking fish eye pictures with little cost and great results. Don't forget you can also make one from a discarded lens, a pair of glasses or even make one for your point-and-shoot camera with the same door viewer piece featured above.
How to make a fish eye lens for a Nikon D-90 Digital SLR for $US16 [Instructables via Zedomax]