No way, this thread is not good. Fentanyl is viable for suicide but certain routes of admission, while being faster, will also leave the bloodstream faster too. You can't just make random assumptions with this kind of stuff and hope for the best. You might end up doing a lot of damage without actually dying.
Buying it from a street dealer without some lab kit to test, for CTB purposes, is just outright a terrible idea. If you can find a supplier on the darknet or such and confirm that they're 99% not a scammer, then it's viable. However it's something you want to research a lot more because of the stuff I said above. If you're afraid of things such as what Dani mentioned, the powder being so strong that you might do some damage or even kill yourself and then accidentally put people close to you at risk who may touch the substance without knowing. There are many many risks when dealing with strong illegit powders. Another option is fentanyl patches, these are a real medicine and probably a bit easier to get hold of than pure fent, I'm certain I read a case of someone who suicided with these patches by simply applying more than the recommended dose. As mentioned by
@VanHeineken here https://sanctioned-suicide.net/threads/suicide-by-fentanyl-patch-overdose.1602/
https://www.researchgate.net/publication/5930713_Suicide_by_Fentanyl
https://en.wikibooks.org/wiki/Suicide/Toxification/Fentanyl
Hope this helps
edit: oh and my personal opinion? I think fent is quite possibly a really good method, people are just afraid of what they don't know and are unfamiliar with. There are innumerable cases of N deaths whereas fent deaths are mostly only ever reported as accidents, it's nowhere near comparable as a suicide drug of choice but with a little bit of research you can decide for yourself.
I worry of putting other people at risk and, more importantly something I never got an answer on when I was researching it back in the day, I worry that if the fent were to act
too quickly then what if I were to suffocate due to respiratory depression before I even passed out?