http://www.esri.com/software/arcgis/arcsde/how-to-buy.html.
You must purchase the ArcSDE 9.1 client SDK from an ESRI vendor. There are instructions on how to purchase the client atFollow the instructions that come with the SDK to install the client to a folder outside of the FDO distribution. Make sure that the folder containing the following DLLs is visible in the %PATH% environment variable .