To quickly locate a particular type of primary source in most online catalogues use the following terms as SUBJECT rather than KEYWORD terms:
Entries for countries and regions are usually subdivided by century. If you are looking for what is available to document general social or intellectual trends in a period search for the country's formal name (Great Britain, not "England") followed by the century designation: Great Britain 15th century. Major events in any century will often have a subheading of their own: Great Britain World War 1914 - 1918.
You can use the subheading Sources with these time period designations as well: Hundred Years War 1339 1453 Sources
When searching in WorldCat your result list will have a set of tabs at the top, two may be of interest for primary sources:
Archival - used for collections of unedited primary sources for individuals, professional societies, or corporations
ArchiveGrid includes over 7 million records describing archival materials, bringing together information about historical documents, personal papers, family histories, and more. With over 1,400 archival institutions represented, ArchiveGrid helps researchers looking for primary source materials held in archives, libraries, museums and historical societies.
"The Archives Portal Europe provides access to information on archival material from different European countries as well as information on archival institutions throughout the continent." (web site). You can limit search to digital items only or to country, archive, sound/image/text object, and date (currently only from 1800 forward).
Links to open access sources of European documents that are "transcribed, reproduced in facsimile, or translated." Arranged by three broad chronological periods and by country/nation-state. Includes documents that shed light on historical, cultural, political, and economic history
Provides online access to digital copies of European primary sources. Search by country, time period, language, subject, or type of source. Access to sources is free, but some sites may require registration.
"Europeana is a single access point to millions of books, paintings, films, museum objects and archival records that have been digitised throughout Europe. It is an authoritative source of information coming from European cultural and scientific institutions." From music to art nouveau, Eastern European wedding ceremonies to musical instruments, this site makes many aspects of European culture accessible to users.
"This archive of primary documents from World War One has been assembled by volunteers of the World War I Military History List (WWI-L). International in focus, the archive intends to present in one location primary documents concerning the Great War. "
Organized into 9 subject/content sections, this site includes a biographical directory, an image archive, and a section on diaries and personal reminiscences, among other sections.