Hiring an assassin
Finding a chemistry lab wasn't so hard—King Lion Gang wasn't producing drugs themselves, but they had a place to check their quality and sort them out before dealing.
Lately, that makeshift lab was being mostly empty because of the police pressure, and the drugs had to stay untested, but so far, your overseas suppliers didn't fuck you over.
As for the ingredients, Tristan bought what he thought he might need with untraceable cryptocurrency.
During that time, he also found something online which, he thought, might help him out later. Not a thing to be bought, but a contact.
A week after he came up with his idea, he was standing in the lab, neck-deep into cooking the poison. The entire thing was known only to the few people who directly helped you with running errands and such.
Another week later, Tristan had his poison, alongside several failed prototypes and a bunch of mice he tested it on.