Collections of primary texts in anthologies can be good for browsing. They touch on many different issues, often excerpting brief passages from full works that you can find in the tricolleges or through interlibrary loan.
There are some online collections available for historical texts. Haverford has access to subscription databases like Early English Books Online (EEBO). Other primary source collections like Gallica from France's Bibliothèque national are open access on the Web.
While some primary source books and other kinds of materials have been digitized, there are great numbers which exist only in print or manuscript, particularly when it comes to documents and records which are often in one unique copy. Visiting a rare book library will give you access to additional source material, as you can readily see from the resources on Friends Hospital here in Haverford's Special Collection Library.
Use the databases below to search for primary source material in special collection libraries.
Still other sources are available in modern printed editions. Like these titles below from the tri-colleges.
To find more primary sources, search in Tripod or WorldCat for these subject headings combined with topical words:
(sources OR "early works") "mental* ill*" = 178 records in WorldCat
Modern editions, facsimiles, and copies of primary sources on microfilm can be borrowed through E Z Borrow or interlibrary loan if they are not in Tripod.