ABI Inform, Business Source Complete, and CBCA 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]). 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).)