MacBook Air :: Letter Wearing Through - Get Just "A" / Replace Full Keyboard?

I have had my MacBook Air for about 18 months. I've been using Macs since 1988 and absolutely love this machine. In the last two years I have used it for the usual stuff, surfing the Net, e-mailing, doing research, blogging, but most of all I have used it to write four novels, each about 400 pages in length. I use my machine at least five hours a day, sometimes sixteen to eighteen hours. This last week the surface of the "A" key has worn through under the cross stroke, showing some white. I'm not in any kind of panic about it. Certainly I have pushed on this key enough to warrant some wear. My question: If I want to replace this key, will I be able to get just the "A" or will I have to have the entire keyboard replaced?