The black market is pretty much word of mouth. Criminals usually know other criminals so if you know one well enough (a drug dealer, prostitute) and ask, they may be able to hook you up.
If you don't know any, you probably won't have any access. Walking up to a hooker or drug dealer and asking about guns raises tons of red flags. You kinda have to incriminate yourself to gain trust and I wouldn't recommend that at all. It's easy to get locked up in the states.