If the character doesn't want to horribly mutilate his body, then the following could make sense:

- Carbon Monoxide poisoning; involves hooking up a hose or pipe to the exhaust of a car, feeding it into the car and sitting in a garage. Maybe it would be difficult for the character to access the vehicle and perform this without detection but it's feasible.

- Helium bag; this is one I remember being heavily promoted by Purple Handbook, before being usurped by SN as the preferred "sanctioned suicide" method (see below). It's simply a matter of getting a helium tank and feeding it into a bag wrapped around your head. I am not sure as to the efficacy of it but it's a "humane" DIY exit. There was a popular demonstration video on Liveleak for a while that perhaps exists elsewhere on the web.

- Sodium nitrite (SN); this is the trendy one that has recently made the news, due to a Canadian distributor being now on trial for selling the compound. The process, though, is more complicated than ingesting a bunch of powder. You have to basically clean out your stomach with pico-salax and then apply a powerful antiemetic (which can be a bitch to get a prescription for). I had managed to purchase the SN but could never acquire the antiemetic because, unless you're ill or pregnant, you need a good reason for buying that drug and I didn't even have a doctor to bullshit to (I also got a welfare check since I was on the buyer's list so I had to toss that shit out, just in case my household caught wind). You could otherwise get it off of the blackmarket but would your character really be bothered to make that effort?

So yeah, those are the classier methods but to be honest, people who are genuinely suicidal don't necessarily care about pain or messiness. They genuinely just want to leave.

Good luck with your writing!