Hello
Unfortunately, this version is part of the Semi-Annual Channel (SAC) releases, which are generally not available through the usual long-term servicing channels like the Volume Licensing Service Center (VLSC) or Visual Studio Subscriber downloads.
How to download windows server 2019 update to 1903 - Microsoft Community Hub
However, you can try the following options:
Microsoft Evaluation Center: You can download an evaluation version of Windows Server 2019, which might include the 1903 update. Keep in mind that this is a time-limited evaluation version.
Windows Server 2019 | Microsoft Evaluation Center
Windows Update: If you have an existing installation of Windows Server 2019, you might be able to update it to version 1903 using Windows Update or the Windows 10 Update Assistant.
If these options don’t work, you might need to consider using a different version of Windows Server that supports WSL 2, or explore other ways to run your desired applications.