We do NOT subscribe to the Harvard Business School Press case studies. (That would be really expensive ...) However, please be aware that these case studies are actually quite cheap to buy individually; they go for about $7 each when you buy them directly from Harvard. Below is a link to their site.
That said, there are some case studies within the journal Harvard Business Review (HBR). These are not the Harvard Business School case studies (the ones used in the Harvard Business School classes); these tend to be shorter, less detailed and less in-depth, and are usually about fictitious companies. Also below is a link to the full-text of the HBR.
Harvard has some recommendations for this. Below is a short video introduction and a book about it.
Read it ONLINE or download individual chapters, that is my recommendation! Downloading the complete ebook is only necessary if you need to read it OFFLINE; be aware that there are several requirements for downloading an ebook.
For these databases, try the following types of searches:
In each of the databases, what happens after you get to a list of results will vary ... if you need help, please ask?!