Here are the legitimate ways to obtain the ISO image. Microsoft maintains a dedicated portal for older software called the Volume Licensing Service Center (VLSC) or the Visual Studio Subscriptions (formerly MSDN) portal. However, for non-subscribers, Microsoft redirects legacy OS downloads to a specific archive site.

Windows Server 2008 R2 reached its End of Life (EOL) on January 14, 2020. Microsoft no longer provides security updates or technical support for this operating system. Using it on a production network connected to the internet is a significant security risk. This information is provided for legacy hardware support, offline lab environments, or archival purposes. How to Download Windows Server 2008 R2 ISO Images (Legacy Guide) If you need to restore an old server, run a legacy application, or set up a virtual lab for testing, you may need the installation media for Windows Server 2008 R2. Unlike modern OS versions, Microsoft does not offer this ISO via a simple public "Download" button on their main website anymore.

Microsoft occasionally still serves the ISO via their "Software Download" site if you have a subscription. Check: https://www.microsoft.com/en-us/software-download/windowsserver2008r2 – but be prepared for a redirect to a newer OS.