I also like working with MongoDB database. I have a limited exposure and experience learning MongoDB and working with it.
Yes, you can find small projects, and gigs for just specific aspects like, for example data querying, on the international freelancing sites. You can try looking up online, and maybe join one of them and try and get a feel of it. Another thing you can consider is, partnering with other freelancers and provide your specialized service in a larger project.