There are two basic questions one can ask about (not) having a DOI - one is, do I always need one? And the other is, what if I can't find one? We'll answer these in order.
1) Do I have to use a DOI? What if it's a print source?
The exact language in the APA Manual, 6th Edition, concerning this point is as follows: "We recommend that when DOIs are available, you include them for both print and electronic sources" (189). This basically obligates you to try to find a DOI for a journal article in all cases. If you can't find the DOI, see 2a) below.
You are also, according to the APA Manual, obligated to attempt to find a DOI for a book. If you find a DOI for a book, that takes the place of the location and publisher at the end of the citation. However, many print books, especially older print books, do not have DOIs assigned to them; in these cases, the location and publisher remain in their usual place at the end of the citation. For e-books with no discoverable DOI, see 2b) below.
2) What if I can't find the DOI?
If there is no DOI in a database (or other) record for a journal article, and http://search.crossref.org also fails to turn up a DOI, then go to the main page of that journal on the publisher website, copy the URL for that main page, and paste that URL following "Retrieved from" in place of the DOI as the final element in the citation. For example:
|Fink, C. K. (2013). The Cultivation of Virtue in Buddhist Ethics. Journal of Buddhist Ethics, 20.
Retrieved from http://blogs.dickinson.edu/buddhistethics
To find the publisher website for the journal you need, you usually can simply Google the name of the journal in quotes, e.g. "Journal of Buddhist Ethics." When the word "journal" is not part of the journal name, it often helps to put the name of the journal in quotes, and the word 'journal' outside of the quotes, e.g. "Explorations in Renaissance Culture" + journal.
For a journal article where the DOI is undiscoverable and either the journal has no website or the article in question is not available electronically on that website, e.g. it's too old, the citation can simply end after the volume, issue and inclusive page numbers with no further information.
If there is no DOI discoverable for a print book, the location and publisher remain in their usual place at the end of the citation. For an e-book with no discoverable DOI, the final element in the citation may be a retrieval statement (e.g. 'Retrieved from http://www.qualitynonfiction.com') rather than either a DOI or a location and publisher.