I'm currently writing a murder mystery in which the killer uses various poisons to claim his victims. The poisons are essential, since the means by which he kills someone turn out to be clues to future victims. Here's my problem, though: through my research, I've found plenty of info on the poisons and chemicals that the killer needs, but I've also found that many of them are either illegal, difficult to find, or both. At least a couple are, in fact, used in the manufacturing of chemical weapons. Is it a cop-out to have a kind of black market dealer who could provide substances like these? I wouldn't be delving to deeply into this, since the novel is a first-person narrative. We wouldn't be seeing the murderer visit this supplier or otherwise get a lot of details on how he obtained these substances, but on the other hand, the supplier could provide a lead on how to catch the killer. Is this too unrealistic? I feel like it's too pat -- as if I'm saying, "But how did this killer (a layman) get his murder weapons? Oh, there's just this guy who can get stuff like that."
The difficulty to obtain them could work... if (without the black market dealer) the killer had to go to great lengths to obtain the poisons, then it would leave a trail of clues that eventually point to a suspect (or can be used as red herrings). I think this could become central to the plot.
The supplier could wager a hefty price/cost for the poisons, or there could be strings attached to the arrangement they have. Hell, the MC could even kill the supplier and try to keep that hidden while using the supplier's remaining goods and contacts. It also depends on the kind of setting you have. You can get scopolamine all over the place in Colombia, but you can't even find good meth in the US anymore, which sucks. (jk, jk, but you really can find scopolamine quite easily in Colombia) Anyway, you could do a lot of different things to add depth and make it believable.
another option is to use poisons that are easier to find - e.g poisonous fungi like destroying angel or death cap, poisonous plans like lily of the valley or oeleander, ricin (which can be extracted from kidney beans reasonably easily), pesticides, rat poison etc
Sounds good Something for the protagonist to to investigating is not a "flaw" in a mystery plot, it is the mystery plot
None of that is a cop out. If you made up a fictional poison... https://en.wikipedia.org/wiki/List_of_fictional_toxins ...then THAT might be a cop out. If the poisons are hard to obtain, perhaps the INGREDIENTS to make the chemicals are easier to get? Methamphetamine, for example, is made from Acetone (found in paint thinner), Lithium (used in batteries), Toluene (found in brake fluid), Pseudoephedrine (Decongestant found in cold medicine), etc... So maybe your killer cooks his own poisons?
