For document symbol numbers that are not from the main bodies...
1) Search for the title (not the symbol number) in the Cornell Library Catalog or Worldcat* The item may have been purchased separately and included in a library's holdings. Example: the Official Records of the Human Rights Committee.
2) Check the Official Web Site locator of the United Nations System of Organizations. (Some of these websites will include documents, although coverage will vary).
3) Search AccessUN. In some cases the working documents of subsidiary bodies or agencies are included as Annexes to the working papers or official records of the the main bodies.
4) Check the Yearbook of the United Nations, 1946 - , for additional clues.
See also: Sales Publications