Several options. Ask a homeless junkie - chances are, they will know someone. Alternatively, find someone who uses weed. Ask them where to buy weed. Ask the person who sells weed where to buy party drugs. If you start out asking for fentanyl, the most likely outcomes are: you will get lectured 'drugs bad, life good, suicide bad, get help you sad cunt', you will be ignored, you will be treated with suspicion or asked if you are with law enforcement, and depending on your response to those suspicions (tip: you're damned if you do, damned if you don't - there is nothing you can say that will ease that paranoia), may be on the receiving end of violence. Even most dark net markets have rules explicitly forbidding the sale of fentanyl and its analogues.
The good news is, fentanyl is in things that aren't sold as such. Aside from heroin, pills are a fine source - all manner of benzodiazepines, ecstasy / MDMA, oxy / hydro-codone / -morphone, amphetamines (though this seems to be rarer). Depending on where you live, it may be more difficult to get actual heroin than to get fentanyl. I know this because my original method was a heroin overdose. Yet none of the stamps I've gotten thus far and tested have contained detectable levels of heroin - but all have contained detectable levels of fentanyl. Keep in mind that much of the drug scene is location dependent. You may have to travel to a nearby metropolitan area or one which is infamous for its drug problem. The internet is your best friend.
Avoid getting your fentanyl from things sold as 'uppers' as the effects of any stimulating drugs can be counterproductive and send you on a trip you may not want to have. If you've advanced chemistry knowledge and access to reagents, you could theoretically set up a means of purifying or isolating your fentanyl from most other active ingredients. The average person who wants to use this method to kill themselves likely has neither the knowledge nor the access - so stick with downers.
Another route is to know a /very trustworthy/ someone in the veterinary or medical field. Or work in the field yourself as a technician. Would need to act quickly with this one, though - I imagine once the authorities figured out a dose was misplaced they would initiate a rigourous search of all parties involved - no time to keep it for a rainy day as with well-sealed N or SN.