Moderation is not possible for an alcoholic, and there is no pill that can allow you to return to moderation unfortunately.
Naltrexone is sometimes subscribed to reduce cravings in those trying to quit, but it's to be used in conjunction with a sobriety plan, just like all other similar drugs. It's not a sobriety plan in itself.
And yes, it is a prescription drug so any physician can prescribe it. That is your best bet ( seeing a doctor ) to evaluate your need for said drugs.