What are the online platforms that cater to freelance data scientists looking for work?
Stackoverflow has a jobs page. This link is to a search with keyword data and option Remote.
freelandr offers you to `Land your next Freelance Job'.
Here are two specialised job agencies, although I don't know if they relay freelance opportunities.


  We are well aware of the data scientist being the sexiest job of the 21st century. JamieAi harnessing this fact connects technical talent with data-oriented jobs organizations of all types and sizes. The start-up firm has combined AI insights and human oversight to reduce hiring costs and eliminate bias.  Also, third party recruitment agencies are removed from the process to boost transparency and efficiency in the path to employment. Another example is Woo.io, a marketplace for matching tech professionals and companies.
  https://hub.packtpub.com/25-startups-machine-learning-differently-2018/

R-users specialises in the language R.