Are you thinking of 1,4-butanediol? That's still sold quite openly in the US, I think. It's converted to GHB in the liver, but the way people describe effects is not the same, like a dirtier version. An overdose would have a high chance of vomiting, so unreliable, and causing a seizure and subsequent vomiting, which adds to unreliability. It's something which could easily kill, but it wouldn't be guaranteed, if that is important to you. This class of drugs causes quite a bit of nausea, although not as much as alcohol or opioids, so finding a lethal combination would be difficult without sacrificing reliability. I still think for most people, IV opioid massive OD after oral benzos is going to be the most reliable, peaceful, budget drug method. Just don't do it near anyone with a naloxone kit.