My first (and only) Suicide Attempt was back in 1989 with carbon monoxide from a car. This was a 1978 gas guzzler that spewed out fumes in a manner unheard of by today's standards. I was able to achieve clinical death, but then I was brought back via CPR (hooray). In retrospect, all I has to do was run a hose from the exhaust to the window of the car. However, I was young and dumb...& not really consciously trying to kill myself (I now know that I was in the midst of autistic burnout, but didn't understand it at the time).
If you can find an old car without a (functional) catalytic converter, this might still be viable. Otherwise, I might suggest a house generator. I have one of these sitting in my garage right now as a backup plan, but it does come with a CO sensor that needs to be bypassed. Word of advice: If you just redo the wiring to bypass the sensor, Generac generators will not shut off for some reason. They just keep going until gas runs out. The on/off switch literally will not work. Found this out the hard way. Still trying to figure out a better method of bypassing. Anyhow, these kick out enough toxic fumes that it should do the job in an enclosed space...without the risk of blowing up the house.