Primary sources on Russian history: resources for searching
Here are just a few online repositories of primary source documents pertaining to 20th and 21st century Russian history. You can search these websites directly for sources that relate to your topics of interest.
Resources for browsing
The following links will point you to directories of primary source collections related to 20th century Soviet and Russian history. These are large directories with a variety of types of sources, and you're probably better off skimming through them to look for interesting content, rather than running a quick keyword search.