This is probably not what you are going to want to hear, but here goes...
I am the office manager for a solo practitioner, and I have always done her billing. The very last thing I want is unsolicited phone calls during office hours when we are busy with patients. And I get quite a few in one week. We don't need a billing service, we don't need a debt collection agency, we don't need a new copy machine, etc. In fact, if I were in the market for any of those services, and numerous others I didn't mention, I would find one on my own, by asking for referrals from other practices, by going online and looking at websites for various companies, contacting the local Chamber of Commerce, etc.
There is truly nothing more annoying than having to put a patient on hold on the phone, or have to leave a patient in a room during a history taking to answer the phone, and find out it is a cold caller. So my suggestion is if you are going to cold call, do so after hours, when you can leave a message on the practice voice mail. That way, you are not interrupting the doctor/staff during valuable patient hours, when what we want to focus on is the patients. It really is the equivalent of how people hate getting telemarketing calls when they are at home.
As far as obtaining a list goes, I have no idea where you could get something like that. What you can do is research the practices you are thinking of calling by looking at their websites, and see if they appear to be truly solo, or are part of a hospital, etc. Probably it is even better if they DON'T have a website, and are solo/smaller, because that means they are more "old school" about things, and have not signed on with a larger entity. Using the phone book, you can probably figure out who is solo/smaller, and who is not.
Finally, and this is just from MY experience, I work for three billing companies(two small ones, one large one) doing remote billing in addition to my full time job. Not one of those billing companies has ever gotten a client from cold calling. It has all been from knowing someone who needs a biller, then onto referrals from satisfied clients, and also from actually stopping in offices and dropping off information about their services. They all gave up cold calling early on, it just didn't get results.