Don't know if this is of much use to you, but here is a list of all the plastic case parts in a MacBook:

Bottom Case;
Top Case (Apple part includes keys);
Display Bezel;
Display Rear Housing (Maybe AKA Clamshell);
Clutch Cover;
Rear Vent;
Left Bezel Scoop;
Right Bezel Scoop;
Left Clutch Cap;
Right Clutch Cap;

It sounds like you are trying to achieve one of the following:

1: Black MacBook with white keys (but don't like the idea of changing out the bottom case - no offence, it IS a pain);

2: White MacBook with black detailing;

If you are after the former, then doing the bottom case is no worse than doing the display rear housing. If you want the latter, then having the entire lid in black with the base in duotone may look a little unbalanced. You might consider keeping some of the lid parts white. Either a black rear housing with white bezel (makes the whole fitting process easier) or vice versa. If you go for changing out the rear housing, this would be a good time to change the colour of your Apple logo. A small piece of acetate does the job nicely, and a nice large sheet can be found for about 50p from any good stationer or artist supply shop.

eBay and the likes of iFixit are certainly good places to look, but I will add that some repair centres may have left over used case parts they are prepared to sell, or if you spot a whole damaged unit sell on eBay, its worth asking the buyer if he needs all of it. He may have bought it for a single part.
Apple Authorised (warranty) Service Providers can get new parts, but they are not supposed to provide them without also fitting them. Not sure if they are allowed to fit parts to models which didn't ship with those exact part numbers (or Apple approved substitutes) or not. I would guess not but I have never really tried to find out for certain either way. Certainly they can't do that on Macs under warranty. And Apple would not knowingly warranty any part fitted to a different Mac.

Best of luck.