Here's a source I use to get new key caps TheBookYard.
If on the other hand you have damaged keyboard (the innards) so a few keys just don't work. Then your in for a big repair job if you can salvage the keyboard or not.
You see the keyboard is the very first part installed, hence it's the last one to come out. Besides Apple really anchored the keyboard down with a ton of screws and/or rivet pins (depending on the model). Then if you are able to get the keyboard off its still a lot of work taking it apart to get to the pads inside to clean them (assuming they are only dirty). If the given key pad set is damaged there is no way to repair it.
Bottom line here its just easier to replace the top case Vs trying to repair or even replace the keyboard (presently they are not available by them selves).