The most likely would be Bengal.
However, it may not be exclusively Bengal. For instance, if troops were transferred from another Presidency, (Bombay (next most likely) or Madras) to take part in the war, they may have brought their own Commissariat staff.
I have an ancestor who was "office staff" in the Bengal Commissariat, who was never in the Army. A family history document says he travelled with the troops to the 1st Afghan War.
The FIBIS Fibiwiki has a page Unattached List which sets out records available. There may well be additional records.http://wiki.fibis.org/index.php/Unattached_List