Harvard Business School does not make full-text case studies available to libraries - these cases are only available for purchase from the Harvard Business Review website.
Business Source Complete database indexes HBS cases: combine your topic and Harvard Business School Cases. The database provides direct links to the cases on the HBR website, where you can purchase them (usually under $10 USD). Business Source Complete contains full-text articles from HBR that are considered case studies.
Also see the Free Case Studies and Buying Case Studies pages of this guide.
Many books contain case studies in their contents. Combine "case studies" along with another search term in a library catalogue keyword search.
Search market research and article databases for case studies by adding the phrase "case studies" to your search.