You can get the Install.ESD file with the latest Windows 10 build by going to Start -> PC Settings -> Update and recovery -> Preview builds section on the installed system.
When upgrading Windows 8 to Windows 8.1, the install.esd file is downloaded from Microsoft repository in a background mode and is saved to the C:\Windows\SoftwareDistribution\Download\ directory. How to get ESD file with Windows 10 image
In fact, the easiest way to get the installation ISO image of Windows 10 and burn it to an USB flash drive or DVD is to use the free Media Creation Tool utility that helps you download the latest version of the Windows 10 image directly from Microsoft servers.