You just gotta network basically. You meet fellow users, exchange numbers, then build relationships with those people. Can be as simple as sending along a phone number for someone who is looking for something. Could be giving someone a ride or breaking them off a piece of whatever you buy/something you have that's good. Then they'll generally help you out, not out of sympathy but because they get something out of it too, it'll appeal to their self-interest.
Those people know people, who know other people, lol, so before long you know a wide network of people who can get you almost anything. That's what I've found anyway.
Then there will always be the person who doesn't care, they'll rip you off anyway. That's fine, but that'll be the last time they do that with me, because my sense of white trash pride will not allow me to ever do business with that person ever again. I've always felt sorry for those who would continue hanging out with and relying on people who had ripped them off in the past.
The bottom line is that there is always the risk that you will get ripped off when it comes to illegal drugs. There's no way to completely mitigate that. Unless your some maniac who is gonna murder someone over a 50-100 dollar drug buy, the best thing you can do is just swear off whoever ripped you off and consider it a fee for learning what kind of person they are
I'm talking about straight-up fake or non-existent drugs, though, like if someone sells you large salt crystals as meth or something. A variation in quality is to be expected to some degree...although not really with meth, imo, which was generally pretty consistently high-quality (for what it is lol), heroin could have a pretty significant variance in quality though