This product is either discontinued or we cannot get that from suppliers.
Synology Spare Part DISKTRAYTypeD5, Disk Tray NAS Accessories, Drive Bay Adapter, Expansion Bays-1, 2.5"/3.5" Bays-1, FF Internal, Drive Bay Width 3.5".
3.5"/2.5" HDD Tray 2/5bay with locker (12~ Series).