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WorldCat lets you search the catalogs of libraries around the world all in one place. It will be useful for your project for finding both primary and secondary sources in the form of books and archival collections. For the Weimar Republic, these might include memoirs, diaries, and compiled papers. Items can often be requested and obtained through the SMU's Inter-Library Loan system
HathiTrust allows SMU users to log in for full access to in-copyright works online. For your purposes, it works much like WorldCat, except you get full-text online access. You can find secondary sources AND primary sources, depending on publication date
If you are working on a topic related to Jewish history, antisemitism, or Nazism, you may find additional sources here.
If you come across any source that you cannot access through SMU, Inter-Library Loan is always an option! If you fill out a request, the library can get whole books mailed here for you to check out (takes up to 1-2 weeks), or can provide copied or scanned versions of journal articles and other shorter sources (usually takes a couple days). The link to the form is below: