I actually called apple about this before and the person who answered didn't really go into detail, but these are basically the points he made:
Chances are low that apple sells rubber feet for anything else but powerbooks.
It's cheaper to get them at home depot (yeah, sure, whatever)
They probably cost about $30 and he doubts they can just be bought from apple.
Hehe, personally I'd just go to a local computer store and sneak some rubbers off the displays there, they don't really stick on them they clip in (like most rubber feet seem to).