It's really important to know that the data is false because, as mentioned previously, a lot of this data includes people who are consensually working in sex work. A lot of the anti-trafficking groups specifically conflate the two so that it seems like it's a bigger problem than it is—not to say that it isn't a big problem.
If you are looking to get data, what you really need to do is build up relationships of trust with communities so that people feel comfortable coming forward, because we can't study something that can't be studied. If it's actually just pushed further and further down, we can't do any of that kind of study.
Really, it comes down to building relationships.