where is the big sur install

Welcome to Apple Support Communities.

It is here: How to get old versions of macOS - Apple Support.

However, unfortunately, there is a current server issue on Apple's side for the How to get old versions of macOS - Apple Support downloads.

I would try again later.

Cheers,

Jack

As right mentioned by previous posting and it appear Apple Servers are still acting up - not functioning today Saturday Dec 11, 2021 @ 08:00 hrs EDT