A primary source is any item from the time period of the event that you are researching. You will probably seek sources using archives (some digitized), newspapers, social media, non-government organizations (NGOs), or government documents.
Primary sources tell the story as experienced by eyewitnesses or others near to events. These sources are invaluable to researchers because they help to depict "what actually happened," or at least what those who were there say that they experienced. Primary sources can include photographs, interviews, novels, speeches, letters, statistical data, material artifacts, or treaties. These sources offer researchers evidence with which to reconstruct the past.
Some questions to keep in mind when examining a primary source are:
Art: Images and Film
Digital Collections on the Web
Here are selections of digital collections available on the web. These collections contain a variety many different source types--textual documents, images, audiovisual--and cover many subjects. Online collections of primary sources often include commentaries, captions, and other text that are themselves *secondary* sources and should be used as such.
An alternative way to find English-language primary sources, rather than looking for translations, is to turn to English publications from the period. Post-WWI, the Western world at large was very interested in what Germany was doing, so you may find these newspapers talking about things relevant to your topics. Remember, you can also research based on political ideas or cultural movements. These things often were having an impact throughout Europe and the West.