How do I find a collaborator for a social science research project if I'm not in school?
There are a lot of universities in and near Boston. They all have websites and the websites usually include lists of faculty and their specialties. Find someone that way and write to them with a proposal and offer to meet with them personally at their convenience. Introduce yourself and give a bit about your ideas, but don't make it too long. 

Even if they don't want to work directly with you, many would be willing to give you advice about how to proceed once they know what your interests are. The larger institutions, with grad students, may even have a way for you to work with one of them. But academics are good at giving advice if nothing else. 

But, your proposal will need to be pretty compelling, since such people are pretty busy with their normal tasks. Some people will just ignore you for that reason. Keep trying.