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, including the British Library
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:
To find a journal in the British Library, use the search box in the top right-hand corner of their homepage. Select the Main catalogue option and run your search:
Sometimes it is easy to see the journal in the results list. At other times, you will need to go on to run an advanced search. Choose this option from the results page:
On the advanced search screen, change Anywhere to Main Title, and as Material type select Journals, as shown below:
To find out if the British Library has the issue you need, select the Details tab and follow the Item Holdings link:
If the issue you need is there, go to the I want this tab:
If there is an option Deliver item to ... the British Library's reading rooms, then the journal is available in hard copy. If the tab says View online and No access to full text online, then an online version is available - but only from the computers in the British Library reading rooms.