The Bobst Library of New York University has an important collection of Yiddish materials, both in the library itself and in the Special Collections and Archives.
The library catalogue can be consulted here.
Most of the Yiddish and Yiddish-related archival materials can be found in the collections of the The Tamiment Library & Robert F. Wagner Labor Archives and can be divided as follows:
Some Yiddish-related files can also be found in the The Fales Library & Special Collections and those of the New York Historical Society.
This search gives you a direct overview of all relevant archival finding aids in the Bobst library.