One reference is listed here at the moment: the Sony VG-C3EM vertical grip, €269.
A grip like this bolts under the body and repeats the shutter release and controls in the portrait position, so a long session in vertical framing no longer means cocking your wrist above the camera. It also balances heavier lenses and extends runtime. Check the listing for the exact body it matches — Sony grips are not interchangeable between models.
