Hey Munsch,
Yes the fillers (blanks) do exist. I know two big Banks here in Canada that have a bunch in old store rooms....lol
From my old days on filers pre 2000 we used to use MK1 Shelves, and I remember that it is highly recommended to make sure you have blanks in due to airflow/cooling and drive longevity. I would recommend getting blanks ASAP. Talk to your local pre sales rep (NetApp) and see what he can do, they are great guys.
Anthony Feigl