just googling...

just some research notes, no big deal...

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.

carbon monoxide. You just fall asleep and die.

air: 20 cc injected in artery with a syringe is lethal 
water: if you drink 10 lt of water at a time, you will die from water poisoning. 
salt: 125 gr 
apple seeds: if you eat chew apple seeds enough, you will be poisoned and die. 
cherry pits: same story 
tobacco: 300 gr leaves into water, set it aside for 6 days, take out the leaves, simmer the water into 2 table spoon, mix into something then drink. 
some chemicals which are easy to access which are fatal: sodium cyanide, strychnine, mercury, carbonmonoxide, butane, helium, nitrogen, potassium chloride, pesticides 

easy to access drugs: aspirine (30+gr), barbiturates, 

mixtures: zyprexa+kolopin+sleeping pills+sodium or potassium citrate+alcohol 

deadly plants and animals: google them. 

Get some scuba gear, including a really heavy weight belt. They say the deeper you go, the better it gets. Not sure where they got their information.

Or, a bottle of helium gas and a plastic. I have it on hearsay that in the absence of CO2, your bod doesn't even realize it is suffocating.