There are buckling spring Model M and M2 keyboards that will use PS/2 or the easily converted AT. Also look for Focus boards (Alps white) or various Cherry boards (usually Cherry black) with either of those connectors. Arbor, I’ve been trying to offload a Tai Hao with Alps clones and an AT plug for a while now.

Quality of boards takes a nose dive once they start coming with “Win” keys, but you can find decent ones even with that. Gotta be more careful if you want anything other than a run of the mill rubber dome though.

If you want new, you could try to contact Unicomp at pckeyboard.com, they may still have some beige ones in the warehouse, and they already offer special orders for other stuff, so at a minimum they won’t think you’re crazy for asking.