I know of no other data source that breaks it down further. There is data published weekly on grain movement relating to barges and rail but that is more anecdotal as to total amount left in the country side. The most likely place is end users who need continued grain until new becomes available especially in locations that normally do not have corn harvested prior Oct 1.
Edit: For a baseline of total grain storage capacity, the Jan stocks report has a figure in it for both on farm and off. For example, the US has a little over 25 billion bu in storage capacity with IA itself having 3.6 billion.