Various ones available depending on tractor model

https://www.shoupparts.com/search-results/?term=monitor%20brackets&page=1

http://www.tractorseats.com/search.aspx?searchterm=monitor%20brackets

ATU200

https://talk.newagtalk.com/classifieds/Classified.aspx?id=102899

https://www.machinerypete.com/other/precision-ag/john-deere/atu-200

https://www.auctiontime.com/listings/farm-attachment/auctions/online/24787289/john-deere-atu200