New ones are available from Unicomp, and I understand they’re quite nice. In addition to this one, I have a “GE Medical” Model M made by Unicomp around 2014. Both of mine have homemade “Soarer’s Converters,” internal for this one, and PS/2 to USB external for the other. I also use the external one for my 90s military keyboard. I do still have a PS/2 port on my motherboard, but it’s nice to be able to add a Windows/Super key.
I found my Model M’s on eBay and waited around until I got the right deal, about USD $45 shipped for each one. Later models are considered less desirable because they’re (slightly) lighter and some of the manufacturing tooling was getting worn out, but by and large they work perfectly and have fewer miles on them. I also love the keycaps on the medical board. That one’s not mine but is basically identical. A pre-made converted is about another $30-$40 or so on eBay. The most reliable seller is a dude making them in his house in the Philippines.
This one was fairly late, like 1998, so it supported “normal” keycodes. I was able to wire up a completely standard “Soarer’s” converter and simply use dupont connectors on the internal header. The cord itself is just a random one lying around, with a cable tie for strain relief.