For those interested, the Index to Yiddish Periodicals, an ongoing project of The Hebrew University of Jerusalem and Beth Shalom Aleichem (Tel-Aviv), is a fabulous resource for those who wish to investigate the wealth of Yiddish periodical literature published in Europe, America, and Israel (Palestine) in the years before and after the Second World War. It comprises approximately 210,000 (!) bibliographic records from about 800 different Yiddish periodicals and can be searched by author, title, source, date, genre, or subject. The website contains a list of the many publications that have been indexed so far, and because the work continues, you should check back often to see what else has been added!
In addition, an article published a little while back by Steven Heller in Imprint, an online design magazine, reports on the digitization, by the New York Public Library, of its ephemera collections, including many fascinating Yiddish theater announcements, posters, and placards. There’s some really cool stuff there, and you can get lost in all the material they have.