You might want to start with your yellow pages / phone book. Phone all studios and ask questions.
Attend social dances and ask the good dancers where they take lessons from.
Don't forget about independent instructors, you might get a better deal with them than a studio. They are harder to find, you basically have to ask dancers.
Attend competitions if you can and talk to people there - specifically the pro/am divisions.
Have a look at dance-forums.com website; there are lots of NY people on that website.