UPDATE Oct, 6th – Windows 10 1809 and Windows Server 2019 have been pulled out for further investigation after issues reported (see https://t.co/WLZS6uC7Px)
Windows 10 1809 update is being rolled out through WindowsUpdate and is already available using the Installation Media tool or MSDN.
Windows Server 2019 is also available on MSDN or in evaluation, additional availability (volume licensing, Azure VM) will come within the next few days.