These databases are a great place to start as they cover a wide range of business-related topics. They provide access to scholarly, peer-reviewed journal articles as well as some non-journal content (company profiles, industry profiles, market research reports, SWOT analyses, country reports, etc.).
When searching for case studies, use the advanced search tool and select "Case Study" as a Document Type. If this option is not available, try including it as a keyword or subject term in your search. e.g., case studies AND management