the only place i have seen them is arcades, ironically.
having an intrest in minature (28mm scale to be precise) moddeling i can tell you it wouldnt be incredibly hard (and alot cheaper than having a run made by a company) to make a mould of the parts of a pushbutton you want brown and then casting them out of resin that has had a brown colour addadative added to it.
apart from that well, you could try painting or dying white ones, but that would probabally yeild dubious results