IMO, I think the best resources of LORs are from the people who know you best. If you worked with a physician, I think you should ask her/him if they would feel comfortable writing a good LOR on your behalf. As for the writers, I think that most places ask for a science professor and a physician. Some schools, such as OSU-COM, VCOM, and LECOM (I can't remember the rest) require a DO LOR and will not make any substitutions. PCOM i know offhand wants a committee LOR also or the Dean of your university. The number of letters varies from school to school. MSUCOM requires 2 LORs from people who know you extremely well and do not necessarily have to be science profs and physicians.
I know of a couple of people who did not know any DO's, so they found some through various methods of cold calling and asked for an interview so that the DO could get to know the applicant and all the relevant material on the CV so that an LOR could be written. With HIPAA, it may be difficult to find a physician to shadow, so you may want to meet with one a couple of times to ask questions, etc.
I worked with physicians and asked for an LOR. I got both an MD and a DO LOR and a couple of science profs from a LONG time ago. My interviewers told me that my LORs were phenomenal and that they just "loved" me. So, I guess you can say that I had some GREAT LORS!
GOOD LUCK!