When I was living in Nanjing, we used to go to the 串儿 vendors. They always had the hook up, especially around universities. 新疆人 would also at times be a good hook up. In Beijing it was always around 三里屯 ,鼓楼,放假 areas. Rumor had it in Beijing that loads of stuff was coming in through the Nigerian embassy, so any one who you think might be connected to that via nationality might help you out with most things you could want. The thing was that the 警察 were said to look the other way as long as stuff was being sold to foreigners only. Shanghai it was all about the French Concession. You could tell who the sellers were the moment you saw them. Xian, Tianjin, Suzou as well as most other second tier cities I suppose, I would recommend going to youth hostel bars or expat bars. in Dali, it was the old grannies on the corners in the historic downtown.