You will never find it without either Gualfenisin or acetaminophen.
Does DXM need either of those to do what it does?
No.
Would it simplify dosing to be sold alone so that laymen didn't have to worry about potentially overdosing on three drugs at once instead of just one?
Yes.
However, from the war on drugs perspective, that makes it "easier to abuse" to achieve it's hallucinogenic side effect. Bundled with acetaminophen or gualfenisin however, you'd have to be a chemist intimately familiar with how to seperate the other two components to distill DXM in any amount with abuse potential, and the naive non-chemist trying to get high will either end up puking their guts out (Gualfenisin OD) or burning out their liver (Tylenol OD, which is exacerbated by alcohol consumption as well).
The Tylenol one is particularly problematic, because acetaminophen is also commonly prescribed with other common multi-drug formulations that people may not realize are additive.
When you take the route of adding a substance that does harm to discourage a pattern of behavior, you are poisoning. Poisoning being the act of artificially and with intent increasing the toxicity of an imbibed substance to disincent some pattern of behavior.
This is actually based on a natural pattern of behavior by the way. There is a mushroom that is generally completely harmless... Until you drink alcohol. Metabolizing the mushroom depletes the supply of the same enzymes that detoxify alcohol (and Tylenol).
https://en.m.wikipedia.org/wiki/Coprinopsis_atramentaria
So to be clear... If you call this mushroom poisonous, and it targets the same enzyme that alcohol does, then adding something like tylenol to something that doesn't need it to do it's job, you are poisoning.
It just happens to be handwaved because in the establishment's mind, those damn druggies aren't worth caring about anyway.
Not a partaker of DXM, but very concerned with the ethical implications, and the adverse contribution to trust in public health measures that this practice entails.