Two places where you can look to find an article's DOI:
The free DOI lookup at crossref.org is an easy-to-use resource for finding a DOI.
Use this lookup page if you have the title of an article and an author name, and you want to find its DOI.
However, note that not all articles have DOIs.
Not all articles have DOIs.
If for these reasons or some other reason you can't find an article's DOI, you can do one of the following things, depending on what information you have or are able to find:
|Bader, A.L. (1938). Frederick Saunders and the early history of the international copyright movement in America.
Library Quarterly 8(1), 25-39. Retrieved from http://www.jstor.org/stable/4302428