I think there was a limited run of those electroplated zinc (and one titanium) keycaps for spacebars, but I believe they have been out of production for a while now. Weren't they like $99 when they were available, though? That's more than many new keyboards!
Still, if you are able to get a zinc (or other metal) spacebar, it should be pretty easy to electroplate whatever you want onto it. I think I'd go for copper, for that verdigris "statue-of-liberty" look after some time.