Apologies if I'm repeating anything already posted here. Most people have probably already seen these but for anyone who hasn't, here are four films I can personally recommend for researchers:

The Pink Room This documentary is about child sex slavery in Cambodia. It’s very hard-hitting. For example, they show the bloodied night clothes of a little girl whose virginity was sold to a customer. But if you want a very real glimpse into what happens there you can watch it here for free. Unfortunately you will need to provide an email address to view it, but there are no problems with the site at all.

Nefarious: Merchant of Souls This documentary has footage shot in over nineteen different countries and features expert analysis from leaders, ex-traffickers and sex slaves. You will get some great insight from this film, for example, how Moldova in eastern Europe is a central hub for human trafficking in the European/ Middle Eastern region. It is not free to view, but you can rent it here on Vimeo for $4.99.

Human Trafficking (2005) Although this film is not a documentary I believe it has been very well-researched and accurately depicts the underworld of human trafficking. You can see Filipino children being loaded onto shipping containers to be taken across the Pacific--seeing this in particularly really hit home to me what is happening to kids around the world. Watch it here for free on YouTube. (Sorry, the quality is not the best).

Detestable Filmed in the US, this documentary records the details of traumatic memories of Satanic Rituals that involve human sacrifice, sexual abuse, and mind-control techniques. This documentary is not free to watch, but you can rent or buy it here on Vimeo.