I'm assuming you mean the clear little silicone/rubber nipple pieces shown in this photo?
[http://www.brunerd.com/blog/wp-content/uploads/IMG_0065.jpg|Picture]
If so then the answer is no, I've never come across these as standalone replacements. On older machines (PowerBooks, iBooks, etc) you could replace just the keyboard but now that it's part of the top case it's a much more expensive part.
Because of that I'll usually try to repair damaged or missing nipples. If it's torn off completely, but you still have the rubber piece, you can reattach it with a very light amount of glue. The main thing is that you want to make sure it's centered, which luckily the scissor assembly kinda helps with by acting as a guide.
Good luck!