Keyboard is all in one piece. If one key does not work, you will have to replace the whole keyboard. Sometimes the issue of keys not working comes from the logic-board. In that case you should get your device a Professional Cleaning and perhaps Logic-Board repair.
Keyboard replacement on most models are very time consuming. There are about a 100 screws that attach the keyboard to the body of a macbook. Sometimes they are not screwed into the body and they are pressed when they were manufactured. In that case, replacement is even harder as you can guess.
Keyboard Replacements are anywhere from $70-$200 in most Macbook Repair shops in Toronto.