It doesn't need to show its looks on its bottom, where no one can see them, so I think you're being
way too fastidious. But used replacement parts are available for anything, including MBPs, and new parts are available from Apple. Apple won't sell most parts to end users, though — one has to pay Apple to install them unless they're installed under warranty, which your bottom panel wouldn't be. So you can look for a used bottom panel in perfect condition and easily install it yourself, or you can pay Apple to install a new one, or you can just try to get over the fact that your MBP has some trivial cosmetic damage that no one knows about except you.
Here are some other parts vendors:
http://www.pbparts.com (parts and repair work)
http://www.dttservice.com (parts and repair work)
http://www.powerbookresq.com (parts and repair work)
http://www.tekserve.com (parts and repair work)
http://www.ifixit.com (parts and excellent, illustrated online take-apart instructions)