ABI Inform and Business Source Complete contain business cases; just enter your search terms and limit the Document Type to Case Study or Business Case. You may want to use the Number Of Pages option to ensure that the case studies that your search retrieves are a certain length (e.g., greater than 4 [pages]) and limit to Full Text to ensure there is more than just an abstract. You may be able to find case studies in other databases by conducting a keyword search for your topic, along with a keyword search for "case stud*" (e.g., IBM AND strategy AND "case stud*").
(The asterisk in the search statement above is a truncation symbol that tells a database to retrieve various endings of the term preceding the asterisk. So, for example, searching for "case stud*" will find articles containing case study (singular) as well as articles containing case studies (plural).)
Emerald Insight also has business case content. Go to Browse our Content / Case Studies / View All and limit to 'Only Content I Have Access To'. There are about 40 cases to browse through; if there is a case we don't have access to, please contact email@example.com and we can investigate options.