Illinois State University does not collection many print UN documents.
However, the University of Illinois at Urbana-Champaign is a UN depository. Many documents from UIUC can be requested through I-Share (an academic library consortium) for free.
If you want to request an item from the U of I or any other I-Share member library. This help video explains how to do so.
Additionally, you will see links to databases that collect electronic documents in the boxes below and to the right.