The New York Society Library has a digitized collection of marriage and death indexes available for free on its website.
The Notices of Marriage and Deaths of Leading New York Families from 1836-1868 were clipped from newspapers by Elizabeth Denning Duer and compiled in three volumes.
The books came to the Library in 1903 as a gift from Duer’s daughter and are currently in the rare book collection. The full contents of these volumes are transcribed on the website along with page images from the actual books, making this a valuable glimpse into old New York and a unique resource for researchers and genealogists. The books can be viewed online or downloaded as a PDF file.
To start searching the index, go to: https://www.nysoclib.org/collection/nyc-marriage-death-notices