You may use a number of the library article databases to look for case studies. Just try adding the terms (often in quotes as "case study") to your search. You can, of course, combine this with other terms relevant to your information needs.
The Harvard Business School makes cases available online, but they are not a part of our current subscription. You can, however, purchase them directly. They vary in price, but often do no exceed $20.00.
Many of the following databases, though, have similar content and would be a good place to start.