If the broken part is just limited to the button half of the keyhead, you can just buy this for way less money:
http://www.****************************************************.com/product/986KEYBUTTON.html?Category_Code=986Keys
But if the other half is broken too, then you will need the whole keyhead. Kind of expensive since you don't need the electronics portion. You might be able to buy a used one on e-bay or something and transfer your electronics over.