This link does a search of BearCat for collections of primary sources on Russian/Soviet history:
Some of the terms used in library catalogs for collections of primary source documents:
SOURCES - this is the most general term used for a collection of primary documents of various kinds, or a guide to locating primary sources. Example catalog search: russia history sources
PERSONAL NARRATIVES - a general subject heading for first-hand accounts
CORRESPONDENCE - collections of letters
DESCRIPTION AND TRAVEL
ARCHIVES - This usually leads to guides or directories of archival collecrtions. Example catalog search: new york state archives
Books and articles may also reproduce primary sources, either as appendixes or within the text. And they may be excellent sources for learning about what primary sources are known to exist.