I know a group of people who sells empty script pads together with the doctors authentication number which needs to be written on the pad.
The main problem with using them is that if you fill it out and go into the pharmacy to get your prescription yourself, the doctor will sooner or later notice the unauthorized prescriptions and as you write your social securoty number on scripts you are not hard to find.
So basicly some people find some dope head and let him go in and get what is prescribed in change for some money or an empity script for himselves. I myself will not do this as I don't like taking advantage of very vulnerable human beings.
They break into doctors offices and hospitals etc to get them and info on the doctor.
It is also starting to be possible to get a hacker send in a script to the pharmacy. In Denmark we have an electronic script server where doctors can prescribe via computer and then you can get your script on every pharmacy in the country as they all have access to the server. But again the same problem with easy identification.