I like TJ's answer. An alternative strategy: Get to know some doctors who treat the patients of interest. Tell them you're doing some research for your product or service. If they seem interested, ask if they would be willing to hand out a form for you during appointments for those patients.

Make sure you have a good idea of what you want the doctors to tell the patients, including that the form is optional, that it's from an external research team, and how it will help the patient if you had the information (you'll be able to use it to improve certain aspects of their condition).

Most people are used to filling out forms at the doctor's office, and as long as you keep it anonymous you won't break confidentiality laws. Just come and collect the forms that have been completed at the end of the week.

I'd be happy to discuss what to do after the results come in.