The simplest way to search for editorials is to add the term "editorials" to your keyword search. For instance, in the database Newspaper Source you can search: "panama canal" and editorial
Often databases will also provide pre-set limiters that can be chosen before you enter your search terms. In Newspaper Source you also could have chosen a Document Type limiter of Editorial. Then searched by the keywords "Panama Canal."
One of the best sources for editorials is Access World News. It covers many more newspapers than other resources. It's also a great source for foreign newspapers, both written in English and in other languages.
To search enter your keywords combined with "editorials" or "opinions."