Google Scholar has most journal articles. If the article has a DOI, as do most articles published in the last decade or so, you can also use the Add Item by Identifier feature. Some magazine articles and book chapters are in Google Scholar, but for the most part, there is no good online database (that I know of) for these.

I often search for the article title, but I also do make a lot of items manually. If your sources are from the computer era (1980s-present) and particularly if they are from Western Europe or North America, then you should be able to find a database for your field that will have them.

If you are using older or non-Western sources (or non-academic ones), then you will find yourself making lots of entries manually. But take heart, it gets easier and faster over time.

Also, if you find a quality database that fills in one of the the weak points identified above (pre-1980s, non-Western, non-academic-but-useful) and Zotero doesn't support it, I consider coverage to be a priority for translator creation and I will try to write a translator for it.

if you are looking for (mainly) French resources and are lucky to have useful stuff for you on http://www.revues.org/ their translator is really good (because they wrote it: http://leo.hypotheses.org/4841 ).
http://persee.fr/web/guest/home also works quite well.
Otherwise I tend to go for the "google the title and then pick whatever big catalog turns up" - usually google scholar. I then check and complete the info.