In the wake of the release of Windows 10 version 1909, Microsoft has made the Setup ISO available for download.
You get it as you did with past releases: Visit the Download Windows 10 website, select the “Download tool now” button under Create Windows 10 installation media, and then download and run the Windows 10 Setup tool. Then, following the steps in the wizard, choose “Create installation media (USB flash drive, DVD, or ISO file) for another PC,” the correct language, edition, and architecture, and then “ISO file.”
I’ve verified that the ISO you download this way is for version 1909 (as opposed to the previous version, 1903), but if you’re worried about it, you can ensure you get a 1909 ISO by using a bizarre method I’ve seen publicized on Bleeping Computer and elsewhere: Basically, you use the Chrome or new Edge developer tools to fake the site into thinking you’re using an iPhone so that it will skip the wizard download and just deliver you the 32-bit or 64-bit version of the Windows 10 version 1909 ISO.
Either way, you should be good to go.