If you're not already doing it you might check on Roadflea daily and you're likely to find a good deal. You can also try a local dealership to see if they have a set off a newer 5 series that a customer traded out for a different set, I've heard of people getting good deals that way.
http://www.roadfly.org/bmw/classifie...ds.php?cat=215
http://www.roadfly.org/bmw/classifie...ds.php?cat=212