I have the one from K & M Manufacturing, I got the tray for it also. It’s a nice bracket. You have to get the one for your monitor on the command arm (7” or 10”).

Are you needing to buy a second display to monitor your planter? I have the same situation on my air seeder, I had a brown box screen laying around and just use that. You can get just the brown box display for a few hundred dollars.  

 https://www.tractorseats.com/p-413-john-deere-r-series-7-monitor-bracket.aspx