Q. How can I find DOI numbers for citation purposes?
Not every article is going to have a DOI number.
This video explains how to find DOIs in Psychinfo: http://www.apa.org/flash/pubs/databases/tutorials/doi/index.aspx
This link: http://www.crossref.org/guestquery/ will help you look up the DOI numbers up easily. Just past your citations into the form at the bottom.