It's difficult to obtain fentanyl these days as most darknet marketplaces have banned the substance from their platforms. You could try buying fentanyl on the street from local dealers, be wary though as these street-sold products are often impure and could potentially result in an uncomfortable death. Depending on where you live you could try bribing a pharmacist/doctor for a prescription.
Sublinguals are probably the easiest way (next to IV if you're capable of doing that), patches work too (try sticking them to the inside of your lip, also you can cook them to extract fentanyl or swallow the patches as a whole).
Lethal dose should be around 3mg (to make sure). Alcohol & grapefruit juice will assist the process.
If you're not able to obtain fentanyl you could try obtaining morphine or oxycodone.
EDIT: There's a whole chapter in the PPH dedicated to (synthetic) opioids. I advise you to check it out.