Easiest way I found to get the driver is to go to http://catalog.update.microsoft.com/v7/site/Home.aspx
and then search for "storagetek". In the search results you'll see "StorageTek - Storage - Sun/StorageTek Library". The 19K one is the 64-bit version. That was the closest thing I could find, and Device Manager reports the changer as a Sun device "working properly" once the driver is installed.
Having said that... check this out from a posting on the AWS Forum yesterday:
"The generic Microsoft drivers have been tested for use with Gateway-VTL and Symantec NetBackup. It's true that when using these drivers the Media Changer device displays as "Unknown Media Changer" in Device Manager, but the backup application should still be able to detect it as the appropriate device type (ex. STK-L700).
Currently, we do not recommend installing any driver but the generic Microsoft driver for use with the Storage Gateway. However, as stated previously I this thread, we are in the process of actively qualifying other backup applications. If we come to find that a particular application works better with another driver, we will be sure to update our documentation to reflect the new recommendation."