The commercial company Maplethrorpe have just released their 2017 modern slavery index. You can view a summary on their website.
For the UK the latest statistics (including trafficking) are on the National Crime Agency website.
Also available online, ILO estimates of global child labour and forced labour worldwide.
The Walk free Foundation produce an annual Modern Slavery Index. It includes country information about what national governments are doing.
As with all data sources remember to consult the methodology section to check what they are comparing.