I would definitely recommend to download the High Sierra from the Mac App Store -- this week or next, while it is still available. Apple sometimes removes prior OS X/Mac OS releases once the new version is officially available. (And there is no way to know in advance which older OS X release Apple will make disappear forever from its publicly available support downloads. Sigh.)
Since the Mojave 10.14 is scheduled for Sep 24 release, it is possible that these next two weeks will be your only chance to easily and officially get 10.13 High Sierra.
Two important thing to note:
(a) as soon as the 5 GB or so download is finished, it will launch the "Install macOS High Sierra.app", and wait for your confirmation. This is normal Apple OS installer behavior. Just cancel that and close the window.
(b) I would also recommend to move the High Sierra install app to a different folder or at least into a sub folder you create for it. Just to ensure it does not decide to launch itself (for your own good, of course... ) ;-)
This way, having downloaded it, you could also move it off and save it on an external hard drive or flash drive. And Apple would even let you re-download it in the future if needed, once it has logged your official download of the OS 10.13.
(Though, I have sometimes run into difficulties with trying to re-download an OS X installer version. So best to save it yourself on external drive.)
(my suggestions in addition to the good suggestion provided by user cruisin .)