There's actually a pretty good range of ultra-wide APS-C lenses now. Canon has a 10-22mm. Nikon has a 10-24mm. Sigma has an 8-16mm, a 10-20mm, as well as circular and diagonal fisheyes. Tokina has a 11-16mm. Tamron has a 10-24mm. There's others. They're very proud of them too, judging by their prices.
I went a cheaper route and got the Samyang 8mm fisheye. When I want rectilinear output, I convert it with hugin.