AFAIK, Apple does not sell individual keycaps directly to consumers. In most cases, if a keycap is worn but the mechanism is intact, you’d likely need to have the keyboard serviced as a whole. Apple Stores or AASPs typically either suggest replacing a whole keyboard or may even bypass keycap repair entirely—so your best bet is to talk to them directly about your situation. Some users have reported that specific service centers might have spare keycaps available or be willing to make an exception if you bring in the discolored or worn-down key yourself. Still, it's not guaranteed—official support is usually focused on bigger repairs rather than cosmetic keycap fixes.
If your keyboard is still covered under AppleCare+, you may have more flexibility, but keep in mind that coverage likely applies to functional damage—not cosmetic wear such as fading letters.