Starting points
Firstly, check you know the journal, magazine or newspaper that the article comes from. You will also need to know which issue the article comes from: either the issue number, or the date, or preferably both. A search in Summon, Google or Google Scholar should give you this information.
Let's take this article as an example:
The first thing to do is to check the SIA journals list for the journal title (not the article title), using this search box:
If this does not find the journal issue you need, there are several more options.
Next, try JSTOR. SIA subscribes to part of the huge JSTOR collection, and these articles are reliably found by Summon. Individual articles in the rest of JSTOR can also be accessed, though on a much more restricted basis. Here's how: use Google or another search engine to access JSTOR. Do not log-in via a JSTOR link on the library website, a library guide or Canvas. Accessing JSTOR that way will only find articles included in the SIA subscription. If you find your article in JSTOR, you can use the Register & Read service to access up to three articles at a time, in each 14-day period.
Some articles are republished as chapters in a book. See the guide to finding book chapters.
Searching online
Please note that any free copy you find online you may not be the definitive version of the article. It could, for instance, be an early version before the piece received its final edit for publication. If in doubt, consult the SIA librarians, and make it clear in your assignment's bibliography which version you have accessed.
Google Scholar is often the best place to start.
The following are also possibilities, though much of the content in them will be found by Google Scholar:
Searching other libraries
When searching other libraries, you need to search for the journal, magazine or newspaper title, not the article title. It is often best to begin your search with either: