I'm not sure I've ever seen a trackpad protector. I suppose you could try using a Palm device's plastic screen protector or something similar, but I'm not sure what that would do for the sensitivity and usability of the trackpad.
I've seen a few protectors but they only come in set (with keyboard or palm protectors) I've heard moshi is great but I just want the trackpad protector
I just got a MacBook and have been thinking about the same thing because I don't want the oil from my fingers to wear down the trackpad and make it all shiny. So, I did a little research and came across the perfect thing: http://www.powersupportusa.com/products/macbook-tpf.php
I'm worried about 2 things though: losing sensitivity in the trackpad and leaving a mark on the screen from the trackpad cover sticking up further than the trackpad did before. I dunno though. It looks like it might be snug enough.