Which fisheye lens should I get for my Canon XSi?

hey have a Canon XSi and had it for awhile but I really want the fisheye lens. I don't know much about camera for I'm a beginner, but I really like the pictures that you can take with a fisheye. but anyways, can anyone suggest a fisheye lens that I can get, why, if it's worth the money, and where I can get it.