Like with all business, there's two ways to find customers.
Location or word of mouth.
That is why there are drug wars. Rival crews compete for prime real estate locations where known drug addicts are used to getting their drugs from. This is more traditional, and doesn't exist as much in our modern society.
The other way is word of mouth. If you have the good stuff at the right prices, they will come back to you and bring their friends.
The drugs sell themselves. except those thugs on street corners that pressure you to buy their crappy mexican pot full of seeds and stems.
The users find them.
i would tell you but your probably a cop
How do they find people to sell to without drawing unwanted attention to themselves?