It is always fun to visit flea markets when travelling. What are the major options for doing this in the New York City area? We may consider travelling (preferrably by public transportation) around an hour to such a spot. (Staying in Union City right across the river.)
Using public transportation your pretty much only bet is visiting New York City, Jersey City, or Hoboken. Everything else you might need to get a car.
If you do a quick search on the map of Manhattan you will get quite a few. There are larger ones like:
And quite a few smaller ones listed.