Backward search
Look at the bibliography and search for other relevant sources there. In the case of articles in specialist databases, you will often find the cited texts under "Cited references" or "References".
Search for these sources and look at their lists of references as well. This way, your collection of relevant literature on your topic will grow quickly.
Please be aware, however, that this method will only allow you to find literature that is older than your original source. Literature that isn’t cited very frequently can also be missed.