Sony 400-800mm f/6-3-8.0 G lens at BHphoto, Amazon, Adorama, FotoErhardt, Fotokoch, Calumet, WexUK.
Here are two new reviews of the lens:
Sony 400-800mm f6.3 – f8 Comprehensive Field Test & Review
Sony FE 400-800mm f/6.3-8.0 One Month Later: The State of the Art as of Q1 2025