Which iMac model are you using? I ask because you posted this in the `iMac (PPC)` forum, and HS will only run on iMacs produced between Late 2009 and Late 2017, the last PPC Mac was produced in 2005.
If your Mac is one of those versions and you have access to the App Store, you can download the High Sierra installer from here. You will also need a 32GB (or larger) USB flash drive, that you have *already deleted all the files from*
You can then follow the instructions here: Create a bootable installer for macOS – Apple Support (UK) to copy the High Sierra files to the flash drive and then install the version of the system. Please note, if you're running a newer version of macOS on the system already, you will need to restore the Mac to factory settings (removing all documents, applications, settings, etc).
Alternatively, you can use Internet Recovery to download the version of macOS came with the Mac from the factory. To do this, power off the Mac, hold Option+Shift+Cmd+R, power on the computer while continuing to hold the keys until you see a spinning globe icon on the screen.