jewish press Wednesday, Luach, Craigslist (try), and the best is to get a broker. It takes time and be prepared to pay premium $ for below mediocre apartments.
The Jewish newspapers don’t advertise any partments in Flatbush, since Flatbush is around Cortelyou Road & Flatbush Avenue, which is not a predominantly Jewish area.
Shauli123 is interested in Flatbush & not in Midwood as per his/her entry on in this coffee room, so should look in a secular newspaper, or at a Brooklyn Estate Agent website.