I am so sorry to hear about your condition. My heart genuinely goes out to you during such a difficult time.
Right now, Mexico has a serious problem with a shortage/lack of certain controlled substances and opiates (methadone, ketamine and others that require a certain ingredient that cannot be obtained for whatever reason.) The pharmacists are frustrated as well, and have no idea when those meds will become available.
However, other opiates (such as oxycontin) are available. Every drug works differently on every person, so I hope one can be found for you.
May I offer my suggestions based on personal experience:
Sam Thelin: he is not a pain management specialist and has absolutely no meaningful training in that area. He is capable of administering an IV with morphine, but is not capable of avoiding elementary errors as he has made catastrophic error(s) in the past with a patient by prescribing conflicting opiates which sent that patient to hell. Last I heard, he does have a license to prescribe opiates. Proceed with caution.
Dr Maytorena: Very well trained and compassionate, but does NOT have the license to prescribe opiates. He studies a lot in how to manage pain and can prescribe non-controlled substances and could be worth speaking to.
Dr. Garcia at Maskaras: he is an internist and not a pain specialist, but has been a physician for several decades and is quite compassionate; he has a license to prescribe controlled substances and appear to have a good foundation, and knows better than to mix certain opiates. He is a compassionate person and so far I have been very happy with his knowledge and service (knowing he is not a pain specialist, but is in town which makes things convenient.)
The Lakeside pain management doc is one that comes to town once a week and while I don't know much about her training, she does have the license to prescribe opiates, but demonstrated to be very unreliable (canceling appts w/no notice to a patient, thus leaving the patient out of meds.)
Dr Lastra is very compassionate and I know he does home visits. However, last I heard he does NOT have the license to legally prescribe opiates. His method is to buy them himself and sell them to you at a markup. That is not legal (although given your situation, that might not be something you understandably care about right now). Maybe he has a license now, but I would ask him first, as w/o a valid prescription on your person, you could get into some trouble, which is the last thing you need.
Again, I'm so very sorry you find yourself in the position you are in. Please contact Dr. Garcia at Maskaras. He has the license, compassion, and there is always a doc on call there. There does not appear to be a decent pain management specialist located here in Ajijic w/the license to prescribe controlled substances, but I hope you can find a doc knowledgeable (and legal) to assist you. A lot will depend on what you need, such as just pills you can take on your own right now, or someone to come to your home and administer an IV? And of course your needs may change over time.
I wish you good luck in finding someone that can assist you.