NO lol, what kinda shit do you think this guy deals in? just because he sells drugs you think he has some legit connection for poison??? How do you even have a drug dealer with this silly skewed perception of what drugs that street level dealers have connections in. No offense. If I was just selling coke or some shit and some depressed dude asked me to source him poison I would have so many reactions, none of which would be "ok, let me just call up my guy."
I mean hell, if you end up asking and he says yes, please let me know because I would be thoroughly mindfucked. Maybe it would be less, odd, to just ask him to source you Fentanyl? I mean, at least he would find that to be a more reasonable question for his line of work.
He's almost definitely a pro-lifer even if he's "shady" because most people are. Even if he isn't, he will almost definitely not be okay with assisting your suicide. Even if he was morally okay with it all, it is a major risk to him legally, and he is already a drug dealer. I guess though, because he probably wants no contact with the authorities what-so-ever, you don't really run the risk of him calling the cops on you and trying to get you sectioned...