Where can I find a particular version of an older raspberry pi firmware?

Ref: Where can I download older version of Raspberry Pi Imager

Given a Raspberry Pi-4 with the latest firmware and an older version of Buster dating back to 2019

I need to run that version of Buster on my raspberry pi for testing purposes but it fails with a “startelf4 is not compatible” and that the “software” is not compatible.

So I tried copying over a newer start4 elf file and, though it booted, I received an immediate kernel panic.

Obviously I need to downgrade the Raspberry Pi firmware to a version compatible with a 2019 release of Buster.

So:

How do I determine which version of firmware to use?
Where do I find this older firmware?