You'd be better of just buying opioids or benzos to combine with the opioids intentionally than trying to find some unknown concentration of laced drugs
I wrote a thread on opioid poisoning:
https://sanctioned-suicide.net/threads/opioid-megathread-overview.138948/#post-2216485
There are far more reliable methods and milage will vary from person to person, but if you're set on this method, this is the thread for you.
Read through it carefully and make a plan that is mindful of dosage, the drugs you are taking, whether the drugs are actually what they are, etc. rather than taking some random pills with who knows that in it.
Access is detailed in the thread.