This requires a prescription in the U.S..
It is not a natural herbal-type treatment. It is an artificial chemical and will likely have long-term side effects and interactions that no one knows about. BUT it is much, much better than alcohol abuse, if that is the alternative.
You might be able to get it from an online overseas pharmacy without a prescription. A lot of these are shady and might just be snatching credit card info. Here is a link to a price comparison of some online pharmacies. The comparison site also may be a scam, by the way:
http://www.pharmacychecker.com/generic/price-comparison/naltrexone/50+mg/
The online prices at that comparison site appear to be a lot higher that the local U.S. pharmacy prices. There might be cheaper online pharmacies that are not on the comparison site. (Comparison and review sites often are often scam sites steering business to a particular retailer.)
Here is a price comparison of U.S. pharmacies:
http://www.goodrx.com/naltrexone
WalMart appears to be cheapest at $37.69 for 30 tablets.
It would likely be cheaper if you have Rx insurance. It is available as a generic, so it might be just a few dollars per month.
Because the online pharmacies are so overpriced, and since this drug is taken daily, it is cheaper to get a prescription. You might have a hard time finding a doctor who knows anything about the drug and is willing to prescribe it. I would confirm that the doctor is willing to prescribe the drug for alcoholism before paying for an office visit.
Fortunately, you can get a list of doctors who already prescribe "Naltrexone" oral 1-per-day tablets or "Vivitrol" (monthly injectable Naltrexone).
Go to https://www.propublica.org/ and search for the name of the drug along with the name of your state. One of the search results will be a list of the top prescribing doctors and how many prescriptions per year that doctor writes. The data is from Medicare prescriptions. The website is buggy, so you might have to try a few times. Some states might not have any doctors listed for a particular drug.
For Vivitrol, you can also search for providers on the manufacturers website here: http://www.vivitrol.com/getstarted/findadoctor
Some of the doctors are Addiction Specialists, but many are primary care providers. Some doctors will be politically correct and require you to participate in some kind of counseling program, especially if it is a money-maker for them. A primary care doctor is less likely to have an interest in a counseling program.
You can also check out who is prescribing other drugs referenced in this thread, such as Baclofen.
Good luck!