Macbook Pro 15" Mid 2010 Keycap problem I am having a problem with my "a" key sticking. The key is clean, but I looked at some diagrams and compared them to my key, and it seems that the top pin on the scissor mechanism has snapped off or is otherwise gone.

I would like to replace just the faulty key. Is that possible? Do I need to replace the keyboard assembly as well? 

Where can I find a replacement? 

I looked at iFixit and they did not have the part. I also looked on PowerbookMedic and they had the entire top keyboard assembly, but I don't want to have to go that route if there is only one faulty key.
Maybe go to your local apple store (or repair center).
They often have spare keys around from broken keyboards and/or topcases.
Otherwise check ebay (make sure it's the same model. Apple changed the mechanism behind they keys 2 or 3 times afaik)