There are two kinds/levels of case studies. Those you find in business journals may be shorter or reviews of actual published case studies. Individually published Case Studies will be longer and more in depth
There are a number of places and ways to search for case studies depending on your field of research. Some general tips are to:
You can search for Harvard Business School Case Studies but due to copyright restrictions, libraries cannot purchase Harvard Business School case studies or put them on reserve. Students can purchase them directly.
Here are a several databases and websites that contain case studies.