Keyboard Part Number or Replacement of Keys

I have a few keys that are worn and I would like to replace them. How do I go about finding a proper fit? I see there are tons sold on eBay and other sites, however most look like OEM China, not Apple and those items do not show me what the plastic key looks like, or which scissors attach (some have the hinge close and some way off). How do I know mine is the right one? Under the keyboard it says 99OR.COA0935 (when I remove the space bar). I don't want to replace the entire keyboard (or do I). Without taking this apart how can I find out the model number or part number? Thanks! I have the 13" 2.26GHz and I love it!