Microsoft no longer directly offers 2008 R2 ISOs on their main site, but you can find older evaluation versions via the (formerly MSDN) or legacy channels if you have an account.

Using this ISO in a production environment is a severe security violation in 2024.

If the server must be used (e.g., to run legacy hardware incompatible with newer OSs), it should be treated as "air-gapped":

dism /online /disable-feature /featurename:ServerCore /remove