If the document you want isn't available online
If it's a published source (Book or Article)
First, try Google to see if there's a digitized copy available elsewhere.
Tip: try putting the exact title in double quotes "like this"
Second, search for the title in Tripod to see if the Tri-Co Libraries have a copy.
Third, try to request a copy via EZBorrow. If there are no copies there, fill out the Interlibrary Loan form to try to request a copy from another library.
If it's an unpublished manuscript
Local collections in the Philadelphia area include awesome collections. When you've found something you'd like to see, contact the archive to make an appointment & take a look at the Guide to Using Archives from the Society of American Archivists for help.
Quick Tips for Searching Complex Websites
If you find a particular website (e.g. Internet Archive or the Library of Congress) difficult to search, try creating your own search engine for that site using Google.
Go to the Google search bar in your browser and enter your search terms (e.g. smallpox vaccination India).
Add the following to the end of your site: site:archive.org (where you replace "archive.org" with the website you want to search)
So your search terms would look like this:
smallpox vaccination India site:archive.org
Major 19th C. U.S. Online Primary Source Collections
Large Collections and Directories of Online Primary Sources
These links point to websites that allow for searching across the collections of many different libraries at once. Some (e.g. Internet Archive) also include content that has been submitted by users.