True fisheye lenses like this Tokina 10-17mm lens attach directly to the dSLR camera. It is expensive but provides the best quality for super wide angle shots. The extreme curve is very evident in the pictures this type of lens produces.
If you can not afford such a lens for your dSLR or if your camera is not a dSLR, your choice is to get a semi-fisheye adapter. It is cheaper but with much poorer optical quality. The angle of view is not as pronounced as true fisheyes but is wide enough for your budget.
To use a semi-fisheye adapter, your existing lens must have a filter thread at its front end. The semi-fisheye can screw on to the existing lens of your camera to get the desired effect.
Now you know!