For a SQL Server 2008 R2 64 bit System I use IBM System Access for Windows V6R1 SI42423 (64-bit) on Windows 2008 R1 64-bit with SQL Server.

As was previously stated you will need an IBM Account to get the drivers.

The following are Microsoft prerequisites for the 64-bit version of IBM iSeries Client v6.1 SI42423.

1. Microsoft Visual C++ 2005 SP1 Redistributable Package (x86)

http://www.microsoft.com/download/en/details.aspx?id=5638

Note - This 32-bit fix needs to be applied even though it is a 64-bit operating system

2. Microsoft Visual C++ 2005 SP1 Redistributable Package (x64)

http://www.microsoft.com/download/en/details.aspx?id=18471

3. Both the 32-bit and 64-bit versions of Microsoft Visual C++ 2005 Service Pack 1 Redistributable Package ATL Security Update

http://www.microsoft.com/download/en/details.aspx?id=14431

Note - download and install both vcredist_x64.exe (64-bit) and vcredist_x86.exe (32-bit) even though it is a 64-bit operating system

When you create the ODBC Connections pay close attention to the settings.

HTH.