@kiwoz the switch is part of the RL-1060 FLEXIBLE BOARD, COMPLETE Sony part number is A2225903A . The switch is a tactile switch and Sony part number for that is 178691451 The complete flex, as well as the individual switch, are available at a multitude of places. Of course, you can always try places like mouser.com or digikey.com and others to see if you can find a switch that will fit/work. That could prove very tedious due to the multitude of switches. You would have to measure the physical dimensions of the switch and then do a search by that.
You will find the SM and included parts list under Documents on this page Sony a6400
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