Where can I download a bootable HighSierra installer image which I can just copy onto a memory stick using dd?

JustAStefan wrote:

I have been unable to find an ISO that I can dd onto an USB memory stick to install MacOS High Sierra onto a mid-2011 iMac.

The images I found in the web either aren't recognized as bootable images when I start the computer and press the Windows key when the start chime sounds, or they fail while booting, giving a text message "No bootable device - insert a boot disk and press any key".

Almost all instructions I found in the web require having a second Mac to prepare the image in some way to make it bootable.

I have real difficulties understanding this, as I only use Linux and FreeBSD.

I do not use Windows or MacOS, and I'd be glad not to have to use that, as I do not have access to computers with these OSes.

I only need to get that iMac running, ideally with MacOS. (Windows boots and installs fine from DVD, but that does not help much...)

So I kindly ask for a link to a bootable installer image that does not require another Mac for making it usable.

Just an image that can be flashed onto an USB drive using dd, and just works.

Any idea?

Under to limited constraints you have outlined above - No possible

Below may not be when you what to read but is factual

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of High Sierra would have to be done on a computer that Qualifies to run High Sierra.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

This does not preclude you from using an Alternative machine to gain access to a Qualifying Apple Computer from a family member, friend or associate.

Have you tried using using Command + Option + R to access Internet Recovery Mode which theoretically should give you access to the online macOS 10.13 High Sierra installer. However, even using this key combination, some Macs may only boot to the online installer for the OS which originally shipped with the Mac from the factory which for a 2011 model may be macOS 10.7 which still should allow you to install macOS (perhaps with a bit of difficulty) where you later upgrade to 10.13. This assumes that macOS 10.12.6+ was previously installed at some point in the past so the system firmware is able to access Internet Recovery Mode. If the Mac originally shipped with OS X 10.6 Snow Leopard recovery DVDs, then you can use that to install Snow Leopard, then patch it to the latest version so you can access & use the 10.11 or possibly 10.13 installer.

How to download and install macOS - Apple Support

Otherwise you would need access to another Mac from 2008 to 2018 so you can download & create a bootable macOS 10.11 or 10.13 USB installer using the link provided by @P. Phillips.

For 10.11 -- generally a Mac from 2008 to 2015

For 10.13 -- generally a Mac from Late 2009 to 2018

Edit: Link for Apple article for reinstalling macOS using Internet Recovery Mode:

How to reinstall macOS - Apple Support

What you would like to have doesn't exist.