This is not an exhaustive list of Yiddish theater resources out there, but merely ones I found especially useful in compiling the information on this website (and all the documents on my hard drive). The NYPL’s Dorot Jewish Division has Yiddish theater research guides (that were in fact my starting point) that point to other resources. And of course there are other hobby-project websites, much like mine, which occupy niches of varying specificity.
Jewish Records Indexing - Poland, hosted by JewishGen, provides birth, marriage, and death records for 19th and early 20th century Poland. There I found birth records for Izaak (born 1897, but not registered until 1914) and Lejb (born 1893), as well as a death record for Rywka (1904).
And, of course, there’s Ancestry.com. If you time your free trial correctly and you are only investigating a small slice of a genealogic tree, you can exhaust this resource within the time limit.
The Library of Congress provides on-premises access to over two dozen historical newspapers through ProQuest. Most valuable were The New York Times and The New York Tribune. I also found references in The Baltimore Sun, The Boston Globe, The Jewish Exponent, and The Jewish Advocate.
Through the Internet Archive, I found copies of The Billboard, Variety, and Radio Daily.
The Brooklyn Public Library gives free access to The Brooklyn Daily Eagle, even for those who don’t live locally. This source contained more play announcements than any other.
The National Library of Israel and Tel-Aviv University maintain a repository of searchable digitized Jewish newspapers from around the world. These include The Forward, The Jewish Sentinel, and Al HaMishmar.
Newspapers.com has a one-week free trial through which I found many reviews of Itzik and his performances in The New York Daily News, principally written by Walter Hartman.
I visited the YIVO Archives in New York in person to view the records of the Yiddish Actors’ Union and the Hebrew Actors’ Union (many letters regarding performances and finances), as well as the papers of Jacob Mestel. Many collections are being digitized (in fact, some were set aside for digitization at the time of my visit and were not accessible), so will become available online in the coming years.
Florida Atlantic University has a vast collection of Jewish sheet music and librettos, including from Yiddish theater productions. Some are not digitized, but they will do so for members of the public at a per-page fee.
The Mayrent Collection at the University of Wisconsin-Madison is perhaps the most extensive collection of Yiddish recordings out there. And most importantly, in accordance with donor Sherry Mayrent’s directives, they really do want people to use it! When I started this project they did not have any of Itzik’s records listed online, but (as suggested on their website) I wrote an email inquiring if any were yet to be uploaded and received a prompt reply with a Google Drive folder of music files and label images. Since then, even more of Itzik’s recordings have been posted on their catalog.
The Rachel Network (a consortium of French libraries of judaica and hebraica) lists a number of Itzik’s recordings. Some are available as MP3s.
Russian-Records.com is a hobbyist site for Russian and Eastern European records from the early 20th century with many thousands of uploaded recordings, metadata, and discussions. It can be browsed without an account, but you may need one if you hope to contact other users.