Where can I study what I really interested in as a postdoc researcher?
As a tenured professor you can choose your research area basically as you want (sometimes there may be some pressure from above to go into specific directions but where I am (Germany) there is freedom of research). However you will have many other things to do which can not always choose (particular courses to lecture, administration, meetings, committees...). So it is not like you can do the research you want to do all day. 

So, staying where area may not be as bad as you think now: You are assigned to project B now but 1. you may ask to also contribute to project A at the same time or 2. do a good job at project B and then ask your advisor to work on the project you would like. 

Personally I don't know anybody who funds his research on some private basis. One guy I know works in some company where his work is a bit close to research and he manages to teach courses at the university and publish papers and books. I don't know if he works part time or not...  In Germany you need the title "Privatdozent" to do so or get a "Lehrauftrag" each term.