The Martynas Mazvydas National Library of Lithuania holds a number of Yiddish materials.
The library catalogue includes a Judaic Catalogue which provides 2463 bibliographic records of printed material in Hebrew and Yiddish, published in Lithuania (from the beginning of Jewish book printing till 1940) and stored at the National Library.
For a general search in the library catalogue see this page (search for example with the word "jidiš" or "yiddish").
The Judaica Catalogue can be separately searched here (select for example "idiš" under the menu "kalba").
A number of Yiddish periodicals from the library's holdings can be consulted online through the Epaveldas - Lithuanian Digital Cultural Heritage website.