Get in touch with J.H. Crawford, a guy who has done a lot of research in carfree topics. He seems to be pretty open to inquiries, and he might have some ideas for you.
Facebook page
Carfree Cities website
Another organization is Undriving:
Facebook page
Website
It used to be fairly common to recruit research subjects with newspaper ads. Maybe craigslist or a facebook group? But talk with your academic advisor about sampling error and selection bias. You want to find a sample that accurately represents the population of carfree people.
Good luck! If you have other questions, let me know. I don't have much recent experience in social science research, but I will help if I can.