Warranty has nothing to do with it. If its damaged you're on the hook for the repair if you bring it into apple. They won't sell you the part to do the repair yourself.
Your only option is to search the web, but because the unibody MBPs are relatively new, you may have a bit of a difficult time locating one. I did an informal search and failed to come up with any. What I did find (non unibody MBPS) bottom case was going for around 50 to 75 bucks just for the part.