On the DW its easy -- go to DH (one of the markets in tor.taxi) and in the left there should be a category selection, scroll to opioids - research chemicals. You'll find protonitazene there.
You have to remember, the people buying these nitazenes are essentially "graduating" from other opioids. They have such high tolerances that they need something stronger. 1 or 2 grams would be more than enough to send any of us back where we came from.
If you look into the reddit subreddits, you'll see people telling others to stay away from them at all costs -- these are recreational users warding off others from taking recreational doses because it is so close to overdose.
If you want to guarantee it, take the benzos. As for what megadose is, consider it like 5-10x what is a heavy dose on PsychonautWiki for your chosen benzo.
This is for Xanax, heavy is 2mg, so take like 10mg+