frozensun said:
How can I download that specific version 1903 / 18362.30 and where to find it?
You can check here: TechBench by WZT (v4.1.1), you can also find the iso via Torrents, if you feel ok trusting those downloads.

You can also download directly using RUFUS and create your own installer, RUFUS will allow you to select some versions. Check this article with full instructions: How to download older ISO versions of Windows 10 - Pureinfotech

There is also a public script available on github described here:

Or you can try again on the Microsoft website, all you need is to boot from linux, or change your user agent, then you will see other options, check this article: How to Download an Official Windows 10 ISO without the Media Creation Tool

Here is another source of information (from tenforums) Where to download an old version of windows 10?

Another option is searching on the Beta Archive.