I think that rubber gasket can be replaced...I just had my window replaced, and the new window didn't come with one, however, it looks like the rubber seal was replaced because the old one was creased in those same areas where the window folds in half. BavAuto doesn't have it listed in parts, but I would call them and ask.
<img src='http://www.bmw-forums.com/forum/uploads/post-46-1094081870.jpg' border='0' alt='user posted image' />
Clear Corners and Sidemarkers
ZKW Euro Ellipsoids w/ DDE Enhanced
Debaffled Airbox w/Green Drop-In High Performance Air Filter