Best Bets to Start Finding Articles:
Getting Started in Business Source Ultimate (link is above)
1. Do a keyword search for "leadership theories"
2. Narrow your search results by using the filters on the left side of the page such as the "subject filters"
3. You could also narrow your search results by publication date using the filters on the left side of the page
4. To limit to Peer-Reviewed articles, on the left side of the search results, check the box for "Scholarly (Peer-Reviewed) Journals"
PsychInfo works the same way (link is above)
Screenshot from Business Source Ultimate:
Search Proquest Newspapers, which contains national newspaper articles (link is above)
1. Example search: Leadership and Elon Musk
2. Full-text of newspaper articles