Check out the websites of professional organizations related to comics or graphic novels. For example, the Comics Studies Society may have links to their own published journals or can recommend other sources. Also, look for publishers' websites. Sometimes, they publish academic works related to the graphic novels they produce, and these could be in the form of journal articles. If you're a student, your school library might have a dedicated section for graphic novels research, with access to relevant journals either in print or electronically.
Online platforms like Google Scholar are also great for finding journal articles on graphic novels. Just type in your keywords, and it will show you a list of relevant articles from different sources. However, be aware that not all the results might be freely accessible. Some may require a subscription or payment to view the full text. Additionally, some specialized websites dedicated to literature or comics might also have links to or even host relevant journal articles.