These are some of the most useful resources to find academic articles to use as citations in supporting the extended essays or the internal assessments. This is especially helpful in history IAs (where it's recommended to have around 10 different sources as a guide).
This site: https://scholar.google.com is just the Google search engine which is filtered to find academic sources. Main uses: sciences.
The site: https://www.ebsco.com is a special site which consists of academic and historical libraries. Main uses: humanities.
The site: https://www.jstor.org is also an academic library, and it allows you to see up to 100 free articles per month. Main uses: all.
The site: http://www.citationmachine.net is an online service that creates citations for you in a range of accepted formats (Chicago, APA…).
Writing these long essays is always difficult, but with a good plan and a backbone of academic articles these become much less daunting. Good luck!