The Wikipedia Library helps editors access reliable sources to improve Wikipedia.
From Library holdings, special collections and archives in the WIkipedia Library:
"While primary source materials are not appropriate sources for Wikipedia, including references to archival collections in an article can be an excellent way to lead interested researchers to rich collections as further resources. However, care should be used when linking to archival collection descriptions or finding aids—if the editor is affiliated with the holding institution they should only include links to collections which would provide the best information about a topic. The editor should take care to include links to other important collections located at other institutions. Links to archival collections can be included in the 'External links' section of an article, and if there are enough links to archival collections, it might be appropriate to call attention to this group of links by adding a subsection titled 'Links to archival collections'"