500mm f/5.6 at BHphoto, Amazon, Adorama, FotoErhardt, Calumet, WexUK.
LensVid reviewed the new Sigma 500mm FE lens and concluded:
The 500mm f/5.6 is one of the most impressive lenses that Sigma developed to date, with a compact and light design, rugged build with all of the new technologies and features available to the company at the moment.
While there are quite a few zoom options available for Sony shooters that are less expensive including the Sony 200-600mm and the Sony 100-400mm that we also worked with (which sells for about the same price as the with a 1.4X teleconverter, making it a 140mm-560mm f/6.7-f/8), we always felt that a non-extending compact and lightweight prime is the better choice, especially for bird shooters (but that is more of a personal choice).