500mm f/5.6 at BHphoto, Amazon, Adorama, FotoErhardt, Calumet, WexUK.
SonyAlphaBlog tested the Sigma 500mm lens and concluded:
The Sigma 500mm F5.6 DG DN Sports (3200 euros) is an excellent lens and a very good news for animal shooters. It provides the ultimate image quality in a light and small form factor, much more usable than a 500mm F4 or 400mm f2.8 or the big telephoto zooms at a very reasonable price of 3200 euros. It can fit in a standard backpack
It AF, optical stabilisation , ergonomics, build quality is excellent. Bokeh , color rendition , distortion, Resistance to flare , level of CA are also excellent
The only weak point is the very long min focusing distance of 3.2m and the limitations imposed by Sony : max 15fps and no TC allowedHighly recommended