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: