Triangular 3-light signal heads are made by Oregon Rail Supply, their part # 107. They do a single-side cantilever support, but I don't know about that symmetrical style.
i thought of these guys but it seems they dont sell them or have them listed under the "platform" signals section. http://traincat2.com/
But NYC wasnt the only one to use these, the ERIE/EL used ones similar to those here in NJ. One was taken down a few years ago when they made the connection to Giants stadium.