Find these journals using Journals A-Z (link below):
Backwards Reference Searching
What it yields:
It looks into the sources that your author referenced. This means the sources have already been vetted by your author for credibility, and you know that they are related to the topic at hand. However, it also means that the sources must have been published before your original source, so you will be retrieving older materials.
How to do it:
Tools to use: