There's a sticky thread about how to search for and buy things off the dark web. I think it's at the top of this forum. That's all I know.
I'd be careful about revealing too much about where you live, though. Telling us which state you're in won't bring you any closer to getting fentanyl, but it might catch the attention of state-level authorities eager to enforce some particular statute or other. I'm not crying too hard over that A.S.H. member from Wisconsin who got arrested some years ago—he didn't even bother to deny that he only stuck around to try and get people to hang themselves on a live feed so he could watch. A scumbag, clearly.
The thing is that he was arrested under this little-known anti-assisted suicide law that was peculiar to Wisconsin, and which could have had a broader application to anyone residing in or posting from that state. Part of the law was vacated in a court battle and part wasn't, and anyway whatever … the point is that people who post in good faith can get sucked in by laws like that too.
I propose that all U.S. posters insist that we live in the State of Permanent Intoxication.