Always through friends of friends...once I got hooked up with a girl Adderall dealer's boyfriend, who dealt weed for just a minute, who then hooked me up to another random chick who dealt weed, and who has the absolute bomb. Lol, this sequence of events is about the only positive I still have from having done amphetamines back in the day.
Also, the first weed dealer I ever knew I just met at a dumbass restaurant job where he was a food server. Good times.
Oh and yeah, randomly as shit met a mushroom dealer at a party a few years ago - it was a complete one-off buy, but the dude came through - I'm pretty sure he stopped after that, he wasn't anybody...
But it always seems like the best ones are ones you meet indirectly...I have a theory about this...actually good/successful dealers can socialize and shit, but they're usually just holed up at their place, so it's usually more common to get all their info/number second-hand from potheads or whatever who rave about their product. They're so good at what they do that they can't just go and hang out with whoever whenever - if they do, they usually have to make plans in advance and shit, make sure everyone they deal to on the regular is going to be covered when they're gone. If you actually just met your dealer at a party or a bar, or wherever out in the open, they still might be the shit, I'm not saying that, but this has just been my experience.