As far as I know, these are not available separately.
I've wondered about using very fine (1000 or 2000 grit) sand or emery paper to "sand" down the top of them to expose some red plastic, but honestly the faded color has never bothered me too much.
It is also possible to dye them.
A third option would be to use less-faded buttons from the back seat of a wrecking-yard car. These often see less sun....