I went to the local military surplus store and poked around their storage sheds of ancient stuff, and found an IBM M2, NMB RT101+ (black 'Space Invaders' switches) and a Moretek MCK-101FX (white ALPS). They had been stored in a dry by not airtight shed, so there was a nice layer of desert dust on them. The lady at the counter gave me a funny look and charged me $3 for all 3.
The M2 is acting like it needs new capacitors, but I think it will be fine with a teardown, caps, and cleaning; it was on the bottom of the pile so it's not really yellowed at all. The other two clearly have desert dust in the switches and I'd probably have to disassemble, clean and reassemble 101 individual ALPS or NMB switches for them to be usable.... and they are quite yellowed... So I don't know what I'll do with them.
At any rate, I think I have enough to keep me busy for a little bit!