Sometimes a systems administrator needs to get specific OS installers, due to compatibility issues. MacOS 10.13 (a.k.a. "High Sierra") introduced a new file system called APFS. Apple also started making firmware updates part of OS updates. These changes can cause significant issues with using imaging tools like Deploy Studio.
Thankfully, the excellent MacOS systems administration blog Krypted.com published a way to download the version right before that. So if you need to set up imaging of Macs, this is the last version you can reliably use.
Use it for now, but start planning a new workflow to maintain your Macs; one that doesn't involve imaging. That isn't supported by Apple any more. For details on that particular challenge, look for presentations by Greg Neagle at the MacSysAdmin conference, such as this one.
Note: If Krypted.com isn't available for some reason, the recommendation was simply to use this link.
Update: If you need a different version, Krypted.com has a newer article that covers a number of other versions.