Maltron does a tremendous amount with disability keyboards and are probably among the best if you're looking for a hardware solution.
Alternately, software allows people to use a full keyboard but select a function key to "mirror" (ie: typing "h" still types "h", but "func-h" types "g"; "func-j" becomes "f"; "func-;" becomes "a"; etc). Some folks have had success using a foot pedal for this mirror key.
Voice-to-text software can also be quite helpful.
On the whole, though, given that the disabled community is a smaller subset of the populace coupled with the fact that every disability is unique to some degree (even if quite similar to many others), the complete solution will need to be custom tailored to some extent. This prevents a lot of economies of scale, which in turn discourages many companies from dabbling in this arena.