You can buy just a new expansion tank for your issue. They are available online or at your local dealer.
Here is one I quickly found on ebay, (but have no experience with the seller)
http://cgi.ebay.com/ebaymotors/BMW-E39-520i-523i-525i-528i-530i-Coolant-Expansion-Tank_W0QQitemZ350096261178QQcmdZViewItem
Thanks
Jared
Service Advisor
Level 3 Certified BMW Service Advisor