I'm not exactly sure what you mean. Is it made of white plastic? If so, it's one of the case components. Depending on location, it might be the clutch cover, or (more likely) the bottom case:
MacBook Clutch Cover
MacBook Lower Case
Installation guides for those two parts are on the respective item pages.
Warning: Replacing bottom case components on an A1181 MacBook is an ugly job. You pretty much have to disassemble the entire bottom half of the computer.
As I look at my own A1181 lower case, I think I see what you're talking about. The grill is separable from the rest of the case - it fits into slots next to the hinges.
That said, I've never seen a grill available for sale as a separate item. I suspect it's included with most resellers' lower cases, although the photos one sees are usually from the battery side, so you can't see the hinge area. Here's an eBay auction listing a photo from the hinge side - you can see the grill clearly:
Macbook A1181 Bottom Case Assembly
You'll have to go to the site to check the photos - thank you, eBay, for not letting civilians link directly.
As iFixit is currently out of stock, you might search for the full bottom cases by Apple part numbers: 922-7382 or 922-7896.