Update: While I did order some cheap generic pairs as suggested by Zach, I also found and ordered just the magnets at a site I should have known to check, Total Element. They had exactly what I was looking for at a seemingly good price. Here’s a link that probably won’t last as long as this post (as the internet ages).
https://totalelement.com/products/surface-mounted-door-window-contact-alarm-sensor-replacement-magnet?variant=43140747854002