If you have a USB stick, you could use your iMac to recreate the macOS High Sierra Recovery partition on the USB device. Download the macOS High Sierra installer from the App Store, then quit it if it opens after downloading. Log into an administrator account, then rename your USB drive to Untitled. Open Terminal and execute this command: (or just copy and paste it into Terminal and hit Return)
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ High\ Sierra.app
Enter your administrator password when prompted (it won't show as you type), then hit Return. Terminal will warn you that it is about to erase the USB stick to create Recovery Mode on there. Press the Y key and hit Return. Terminal will proceed to turn your USB stick into a bootable installer for macOS High Sierra. (It can take some time.) When it is completely done, restart your iMac while holding the Option key. You should be prompted to select a startup disk. Use the arrow keys to select Install macOS High Sierra, then hit Return. Your iMac will now boot Recovery Mode for macOS High Sierra. When the Utilities menu appears, choose Reinstall macOS (or Install macOS). You will then be guided through the steps for installing macOS High Sierra onto your iMac.
Hope this helps!
