So far as I know, there is nothing specifically designed for touchpads. However, you may be able to adapt another part.
Virtually all laptop touchpads these days are capacitive sensor devices. Conveniently, so are the touchscreens of the iPod touch and iPhone. This suggests that the flood of iPhone screen protectors that have hit the market should work just fine on your touchpad. You'll probably need to cut one down a bit to get the fit right; but they are common and cheap.
I'm not 100% sure that this will work; but I've never heard of a purpose made touchpad cover, and this seems like by far the easiest and most plausible alternative.
