Many primary sources that are available online are archival or Special Collections sources
which have been digitized and made available by those institutions.
BUT:
Not every source is available online. Most Special Collections/archival libraries are not able to digitize all of their sources (collections) or make them publicly available.
Not every individual item is listed in a collection's finding aid. Many Special Collections/archival libraries do not catalog their collections by individual item. Instead they provide descriptions to the box or folder level.
Not every collection will have an online finding aid. While most institutions are working to get finding aids online, this is an ongoing process for many organizations. You may need to contact an archivist to learn more about which finding aids are available online, and which are not. Some archives will create catalog records for unprocessed collections as a means to signal their existence. Further, research/subject guides may also list unprocessed collections.
Not every library (including archives and Special Collections libraries) is registered with WorldCat. WorldCat and ArchiveGrid (which draws on WorldCat) include information from many, many libraries, but do not include materials from every library/repository.
Not every repository uses standardized descriptive methods. While the majority of archives adhere to professional descriptive standards, some do not. This can make it more difficult to find materials using WorldCat or other similar databases/catalogs.
Most archival/Special Collections libraries will not lend their materials. Due to rareness, fragility, or other restrictions, most items in these kinds of libraries are not available for Interlibrary Loan. Researchers may be able to request that copies of relevant records be made available through Interlibrary Loan, however there may be a charge.
If the repository is able to offer you reproductions (photocopies, PDFs, or audiovisual materials) of the materials you want to look at, expect to pay a fee. Typically there will be a charge for ordering reproductions, often including charging by the page.
Most archival sources are in their original language. Archives and Special Collections libraries do not typically offer translations of their materials. Other scholars or editors may have published or otherwise made available translations of materials.
Your local repositories may not have the archival resources you need. Archives and Special Collections libraries tend to collect deeply in specific areas, rather than widely in many areas. Smaller repositories may also focus on collecting materials relevant to their local community.
Reasons to contact a repository's archivist before planning a visit:
Archival research typically involves one or more of these options:
Contact the repository directly (contact information is generally available on a repository's website) to learn more about:
Archivists (including Dickinson's archivists!) are happy to help orient you to their repository, provide information about their holdings,
and assist you with reproduction orders where available.