The Historical Society of the Westburys is headquartered at:
895 Jericho Turnpike
Westbury, NY 11590
United States
The original answer noted "not located in Westbury, New York but in Westbury, Long Island." This reflects a postal vs. municipal distinction: Westbury is a village in Nassau County on Long Island. "Westbury, New York" and "Westbury, Long Island" refer to the same place—Long Island is the geographic island; New York is the state.
The society sits where the village began—on the old Jericho Turnpike, the colonial road connecting Jamaica (Queens) to the North Shore.