Well there aren't too many options, do you want rectilinear or fisheye? What is your budget?
The only ones I can think of are:
Canon 15mm f/2.8 fisheye (Discontinued, but you can still find them used for around $500)
Sigma 15mm f/2.8 fisheye (I've heard nothing but great things, around $600)
Zeiss 15mm f/2.8 (amazing lens, but $3000)
Canon 8-15mm f/4L fisheye ($1300 or so, again a great lens)