I know this is a late post, but I just wanted to add my experience to this. I can recall one day about four years ago. And several times before that. I can find it anywhere, Hell if I was a cop I would make a bust all the time. It's not that difficult as long as you believe in yourself and stay focused on your mission. I was also pretty drunk, alcohol has a tendency to make people do some bizarre things. Basically what I did, I went to bar after bar carrying on random conversations with people who would have a high probability that they would have weed on them. I'll first start up a conversation, and then lead into a conversation on drugs or weed or something of a sort.
Now, when you think about it, if you see someone coming into a bar half drunk off there ass, and asking about getting some weed. It'll be very unlikely and highly improbable that it's a cop. Being drunk along with other drunks has a tendency to build a rapoor with them. So I just kept this up from bar to bar untill I find someone to do business. Of course this is risky because you may come across a off duty cop, who may want to put you in jail for the fun of it.
But that's why I go to bars located in bad neighborhoods. The type of neighborhoods were off-duty cops would not hang out at. This increases your odds of not running into an off-duty cop. The key thing is to remember is, be half drunk off your ass! Then people will believe you. Also be wary that they might try to take advantage of you, so be smart.