Typically, we try not to comment on stuff like "the law" and the ethics of doctor shopping, but I feel like a Public Service Announcement is probably in order.
In the past, it was pretty easy to go from doctor to doctor. People made easy livings by doctor shopping. Those days are over. Anyone who is thinking about trying to be deceptive with their prescriber needs to reconsider. Records are almost entirely electronic and when you get to the new doctor, you better believe he is going to want to know the motivation behind the switch and will almost certainly review your entire case before prescribing.
You cannot get away with this shit. In the developed world, there are now extensive systems solely dedicated to rooting out doctor shopping and prescription forgery. Sure, maybe you'll get lucky, but if you don't, you could earn a permanent red flag on your file. It is better to not be deceptive. Doctors, like anyone else, do not like being lied to. I've found, if a doc believes I'm deceiving them in any way, the relationship utterly breaks down.
Your best bet is always to be brutally honest. Tell them you're a drug addict, you're trying to get better and "can you help?". That is the best course of action, I assure you. It can help forge a strong bond with your prescriber once they know that you are being honest with them.