Positive terminal cover-engine bay
 

Is the black half cover available as separate part number?
Where to get it?

http://i64.tinypic.com/sov8kg.png

Realoem.com is your friend...

Put in your VIN or make and year to be sure. I used mine. #5 in the diagram:
RealOEM.com - Online BMW Parts Catalog

I can't believe that thing is listed for $16.45. My experience has been the prices at my dealer are higher than what's in realoem, so I'm sure that $16.45 is actually low. jeez.

It appears there is no separate part number for the black color half cover, it comes all together with metal body as well.

I get 16% discount (highest you can get here) at my dealer. after discount dealer prices match realoem.com exactly almost 90% of the time.