A crude but quick fix solution for prototyping: Assuming you can get hold of old faulty PC keyboards; use their "shift" and "enter" keytops to replace the regular keytops on a working PC keyboard. Now in software combine the mechanically connected keys into groups. This may be cheap enough and even customized production could be possible at reasonable price; some PC keyboard manufacturers even sell their keyboards in kit form, albeit without warranty.
Alternately you could salvage big keys from shop/table top calculators.