If you are looking for case studies related to business, the Business Source Premier database is a useful resource. Now you can just type case studies in the search box, but one thing you should notice is that while you get a lot of results (over 129,000 as of March 2014), those results can be all over the place. Here are a few tips for focusing your search:
Put your terms in quotation marks: a search on "case studies" returns about 33,900 results. Using quotation marks forces the database to find uses of the phrase, not just random instances of the words case and studies.
Use Advanced Search and the select a field drop down list. Choose SU Subject Terms. Searching for "case studies" as a subject term results in about 15,900 items.
Use Advanced Search and add a second concept. If you're looking for particular types of case studies, say for example those related to business ethics, include business ethics in the search. Just remember that including or excluding quotation marks will affect the number of references you'll have to review.
The way you ask your question always determines your answer, when it comes to databases; and it's preferable to review a shorter list of relevant articles than a long list that's all over the map.