If it's the master cylinder I think it is then eBay had a genuine one last week for £10 - I nearly bought it! Ebay is probably your best bet - I got a new one for 99p! It wasn't the right one for my car but was an Audi one - it fitted and works fine, just had to bend the pipes slightly.
http://cgi.ebay.co.uk/New-Lockheed-B...QQcmdZViewItem