I guess I see case studies as just more lengthy and specific testimonials.

I have gotten these in the past by asking existing customers to take a survey (I use SurveyMonkey) that asks specific questions. I incent them with a freebie of some type. In the last survey question you want to get them to give you permission to use their responses in your marketing materials and to reformat their responses into a testimonial/case study.

SurveyMonkey works great as a depository for this feedback as well if you are ever wanting to enhance your product or are audited for your testimonials by a regulator or a network.