for EOS30, the battery door can leave dangling there while u mount the battery grip.
for D30/D60/10D/20D/300D/350D, the battery door got the remove from the camera, and slot it inside the grip for safe keeping
anyway, i dun think anyone will have extra "door" to sell away, do let us know once u find out the price from Canon (just curious to find out, even my camera dont have any battery door)