If there was one, most of us wouldn't be here. N is the most reliable method but it's unavailable unless you go to South America, other sources are all scams. (There was one on a DNM last month but it was very likely a scam and it was taken down quickly.)
Phenobarbital and Primidone are available on DNMs but it takes a much longer time to kill you (24h), so you gotta make sure there's no one who can find you in the process.
Unfortunately you would probably pass out before injecting all you need to overdose. They're not stupidly hard to get if you spend time on DNMs though. As for the method, boofing (very good bioavailabilty) or snorting (much less efficient) would do the job, you wouldn't pass out before doing what it takes to CTB.