Currently got this one on my work laptop. Model M terminal board with internal converter. The only layout changes I made versus a normal 102-key are that RCtrl is is a Windows key, and the four keys along the right side of the numpad are =, -, +, and the normal Enter.
Where do I get one of these and how do I make it work with modern USB?
Worth every. Goddamn. Penny.
https://www.pckeyboard.com/page/product/NEW_M
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 converter is about another $30-$40 or so on eBay. The most reliable seller is a dude making them in his house in the Philippines.
Both Unicomp and modelfkeyboards.com are highly recommended.