The Windows Installer documentation on MSDN always lists the download page for the Installer SDK. It's the same location as the platform SDK. Our parent MSDN documentation page that provides links is: Other Sources of Windows Installer Information
Another way of locating the SDK is to look in the MSDN topics for the individual tools. For example, you'll find that the Orca.exe MSDN topic provides you with the link to the download location. Note: by traversing through Windows Installer documentation, it will always be direct to the latest Platform SDK site. As of now (Windows Installer 3.1 release), the latest for Windows Installer for the Server SP1 SDK.
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