Apple - How to download macOS High Sierra from the Mojave App Store?
In previous versions of the beta this was not possible. But in the latest release (as of 9/15/18) it is possible to use direct links to open the download page.
This links can be found on Apple's How To Download pages.
Here are the direct links for a some of them:
All other OS can be purchased from Apple directly and are not in the App Store.
I would elaborate each of the steps given below, but this would be difficult without knowing more about your current partitioning and the model/year of your Mac.
Shrink the partition containing Mojava and add a new
Boot to internet recovery and install the version of macOS that came with your Mac.
If necessary, upgrade the current macOS.
Download High Sierra from the App Store.
(Optional) Remove the new partition and reclaim the free space.
The erase and install process is always an option since the recovery HD will offer to download the latest shipping OS. You could always connect an external USB drive and boot to internet recovery and install a shipping OS to the external - leaving Mojave running on the main storage. Then you'll have a shipping version of the OS to download the installer if that's really what you need instead of just a quick wipe and reinstall.