Yeah, Fentanyl is a good method. In the US a lot of opioid drugs like oxycodon, heroin and sometimes even Xanax (a benzo) are laced with Fentanyl.
You guys have a lot of pressed pills over there that are laced with Fentanyl.
There are no real sources because those things are mostly aquired illegaly by a drug dealer.
When it comes to dark web sites. Most markets actually still forbid the sale of Fentanyl but I think that might change soon, thus making it easier to aquire for people who have the knowledge to use dnm's.
What one could do tho, is directly ask a dealer for Fentanyl or if they could lace another product with it, which is already happening a lot.
I wouldn't recommend using the darknet tho cause obviously it is illegal and you could really fuck yourself up if you make any mistake.
For example if you live in the US there are a lot of pressed oxys with fent in them and accidental overdose occurs frequently.
Fentanyl also exists in europe but I feel like is less common. Opioid overdose is also my preferred method.