Nitrous oxide is painless and will produce unconsciousness in a few seconds. It can be obtained from Auto speed shops. It does require a mask or some way to keep breathing it after one passes out. Nitrous oxide is not poisonous but it will not support life and will cause suffocation if one keeps breathing it after one passes out. To insure death without possible resuscitation and brain damage, one should breathe it for 15 to 20 minutes to insure death.
One way to do this is to hook a gas mask or oxygen mask to a garbage bag full of the gas.
When dentists use this gas for painless dentistry they mix the gas with oxygen to prevent asphyxiation.
The gas can also be obtained from whippets which are tiny tanks of nitrous oxide for making whipped cream. It will take a few of these and a whipped cream maker but you can fill a garbage bag with the gas from them. Also canned whip cream has this gas in them and enough can be obtained from them to fill the garbage bag also.
When kids get high from breathing whippets they usually don't breath enough to pass out or they pass out and drop the whippet and begin breathing air again so the don't die.