smoking tar is a terrible (health & mind) way to medicate for pain. I realize your issue isn't with whether you need pain killers or not (although FYI in the US it's incredibly easy to get prescribed opiates, especially if you can prove you need them) but that you won't have health insurance to cover this.
Your question is actually very common, most people find themselves without healthcare at some point in their lives or another, and it's especially tough for those with serious conditions, like chronic pain. I've known people that were in pain and had no healthcare and they either bought heroin, or went to a methadone clinic and faked an addiction.
But consider this, ~!$10-20 for heroin (and this will only go up due to tolerance), or the ~$10 a day for methadone will add up, fast. This is around $300/ month and at this point it's time to do the math, what's cheaper? the doctors appointment and the prescription with no insurance, or the opiates obtained on the street or at a cash-only clinic (most, if not all methadone clinics are cash only, at least in my area).
You can't put a price on your health, and those with disabilities have an extra hard time just getting by, but there are some programs you should look into like Disability/Welfare type options which help many Americans without insurance have some money to pay for doctors visits and prescriptions. Bottom line, if you have real pain that would cause you to buy heroin off the street to get by, chances are there is a program in the US available to you to help you get health care. But of course, there are sad exceptions to this... not helped by the current state of our economy/legislature/healthcare laws.