@jdb2 I also bought a Nokia N900 in 2019 and asked the same question a week ago
It looks like we cannot have OTA updates with the mirrors but I did not get the technical reason behind it so I guess the OTA updates were done through another process with hardcoded Nokia server names and not using the customizable repositories from the Hildon Application Manager (nobody has confirmed it yet).
If you want to flash, you can find the download links in the thread I started: http://talk.maemo.org/showthread.php?t=100762