Strategies to Consider
Remember that our catalog is old -- in the best sense of the word. You'll find a treasure trove of primary source documents there from all periods, in all languages, and from most parts of the world.
Think about time frame.
One easy way to find texts and other items that are roughly contemporaneous with your course readings is to modify a HOLLIS search you've run, using the date limiters that appear on the right hand side of the screen.
Load your linguistic dice.
Adding the word sources to a keyword search can be useful to find republished collections of primary sources. Reader, anthology, documents or documentary can work well.
Think in terms of genre.
Instead of adding a general word like "sources," run your keyword search in HOLLIS. Then look for the Form/Genre filter on the left side of the results screen.
It's here you'll often find the richest variety of primarysources. Form/genre is where you'll see primary sources of these types (for example): correspondence (the official way of describing letters); diaries; exhibitions; speeches; memoirs; notebooks; personal narratives; pictorial works (a traditional way of identifying a collection of images); photographs.
Scour finding aids.
Manuscripts that are held by Harvard libraries, like Houghton, will usually have an online finding aid linked to their HOLLIS records. Finding aids are detailed item-by-item descriptions of everything in a particular collection. Typically, finding aids will also provide contextual information, like biography, scope/content notes, preferred citation methods, etc. Finding aid URLs appear below the title in a HOLLIS manuscript record.
Think backward from a secondary source.
Remember that the secondary literature you find (scholarly journals and books) will themselves be built on primary source materials.
Canvass the bibliographies and footnotes; if the primary documents exist in a published form (rather than being unique to an archive you may not have access to), consider tracking them down at Harvard (if you're close to Cambridge) or (if you're not) at a library near you (via WorldCat).