My 20s are a blur because I was always high on something. I was very bad off. Of course that was over 10 years ago. All of my friends from then are either clean and have families and/or lives now, dead, or in jail. Or I just lost track of them because I disappeared. And, of course the scene has changed so much. As for real life, yeah you'd have to figure out where the addicts in your town usually congregate and take on the risk of hanging around those areas. Once you start observing, and once people start observing you observing, there's a chance you could get a hook up. But there's also a chance that you could get robbed, kidnapped, shot, etc. It comes with the territory. Lots of not good shit happened to me and my friends back then. But I could always get drugs one way or another.
As for the dark web, download the Tor browser and download a VPN (using a VPN is a good idea in general) to make sure everything is as secure as possible. Most anything you buy on the dark web, you are going to have to pay in bitcoin, so you have to exchange whatever your currency is to bitcoin. If you happen to find something, make sure it doesn't have to go through customs or any other extra barriers and never use other mail services besides whatever your government uses. For example, make sure it's USPS if you're in the United States. Never use a store/service that ships with UPS or Fed Ex in the US. They are private companies so they make their own policies and are not bound by laws that the USPS is. Not that that the US government is actually even pretending to follow laws right now, but I digress. Honestly, the US government has such a hard-on for "radical left terrorists" right now, I wouldn't be surprised if they started going through peoples' mail. Yes, I'm paranoid so you should probably just ignore me. As for determining if a seller is reputable or not, I have no idea. And I have no idea how to determine if they are actually going to send you what they say they will so you'd probably have to have a way to test it.
IV opiates are the way I want to go though. So badly that I would gladly take on the risks of hanging around in dangerous areas in an attempt to get my hands on fent or heroin if I had a car to to go driving around looking for areas I think I might could find some, but I live to far away from anywhere to go walk. I'm to scared to try the dark web though. Not willing to risk prison.