http://www.crossref.org/guestquery/

One of my colleagues shared this wonderful site with his fellow students including me. A major change from the APA 5th to the APA 6th is the use of DOI. Unless the article doesn't have DOI originally, scholars using the APA 6th manual should provide each article's DOI.

I have several manuscripts that are developed more than a year ago. So, finding the DOIs was dounting task for me at least in my mind. (I didn't start it, yet, thankfully.) I think this site will save a lot of my time, because I don't need to re-visit each article's journal database. 

If you are in the same boat with me, give it a try!