You have two choices:

• Download Microsoft's own JDBC driver from: http://msdn.microsoft.com/en-us/sqlserver/aa937724.aspx
  
• Download the jTDS JDBC driver from: http://jtds.sourceforge.net/
  

Previously the jTDS driver was preferred to the older deprecated Microsoft JDBC driver, but recently Microsoft has upgraded the JDBC driver with bug fixes and some performance enhancements.