Do you know where your great-grandmother married? The reason I recommend getting your great-grandmother's marriage record is "theoretically she is the one providing the information" when she got married, and most likely she would know what her mother's maiden was. Once you get your great-grandmother's marriage record (hopefully) it will list her mother's surname and the search MAY be easier to locate the record of your great-great grandfather's marriage record. The problem with getting your great-great grandfather's marriage record is you don't have a date, nor a surname for her, or a location...makes searching difficult and costly.
In New York, the marriage records are $22.00 and they will search in 3 year increments if you don't have an exact date but a time frame, say 1880-1883. However if you don't have the exact date and if they have to do a wide search, the cost goes up exponentially.
I hope this helps,