Is it over the counter or prescribed?
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.