Hahahaha sorry I don't mean to laugh but I've worked in outreach with plenty of people who use and sell drugs and honestly you can just ask for heroin; people know what it is. Some people use slang terms like "down", at least here in Canada. But people still know what heroin is. Anyone involved in the trade is just looking to make quick cash selling dope and will pass it off to you in an instant if you have the cash for it, regardless of what you call it.
Albeit, heroin is rare nowadays (probably less than 1% of the opioids going around, if that) as fentanyl has massively saturated the market. Your odds of fiding heroin in-person (I.e., not on the dark net) is very low. But fentanyl would be just as effective for ctb in the right dose. I wouldn't recommend this method as the concentration of fentanyl in a given sample varies substantially (usually around 5-25% but of course it could be anything). Fentanyl test strips can test for the presence of fentanyl and FTIR analysis (e.g., mail services such as
https://getyourdrugstested.com/) can tell you approximate concentrations (usually within a 10% margin of error), but there's no way to know the exact strength.
Energy Control is another drug checking service in EU:
https://energycontrol-international.org/drug-testing-service/
I would NOT trust anything off the street or that has not been tested using FTIR spectroscopy services (easily accessible via mail/online) given the wide variability in concentration and contamination with other drugs. This site has data on the various contaminants in the opioid supply:
https://drugsdata.org/results.php?search_field=substance&s=heroin
Most of the "white powders" have caffeine, xylazine (inert sugars), varying amounts of fentanyl, trace amounts of heroin, some with cocaine and lidocaine, etc. etc. mixed in. The drug supply especially on the street is very unreliable.
Individual tolerances will also vary substantially.
The best bet if one were set on opioid overdose to ctb would be to learn how to use the dark net, buy some high-quality #4 china white heroin (likely expensive), mail it to an online drug checking service to ensure it is a high concentration, then take some absurd dose that is well above the LD99 (the median dose at which 99% of people die) preferably by intravenous injection (if experienced) or rectal administration (if inexperienced). 100mg of a sample with 30% heroin will have 30mg of heroin, so you will have to do approximate math to ensure your dose is well above the LD99.
I wouldn't recommend smoking or snorting as the pharmacokinetics are too variable and you can waste a lot of the substance meaning more potential to under-dose. Likewise with eating it as you may vomit and the bioavailability is low.
The greatest concern for myself would be traumatic brain injury from non-lethal respiratory depression if I didn't do it right... SN seems to me to be lower-risk in this capacity.