They produce them to spec? I wonder if they can do
small quantities.
Yep, but I don't know about small quantities. I did it for a product
some years ago, but I keep comunicating with the factory. They can even
inject 2-part keys in the color you want. Think high-quality keys!
http://www.fourth.com.br/ if you're curious. Page in portuguese, use
google translator
Another option might be those keyswitches that have
transparent keycaps,
under which you can stick labels.
For prototypes and one-offs I usually build the keyboard with old keys
or entire keyboards.
Of course I adore those keyswitches that have a tiny
color OLED display
in them. At $50+/ea that'd make a pretty expensive calculator though.
It will take some time to drop the price. I'd be more than happy with
any kind of e-Paper key.