The best way to find primary sources is to find secondary sources that discuss your topic, and look in their footnotes/endnotes for who THEY cite. For example, I've got Andrew Roberts' "Napoleon: A Life" on my shelf. There's no entry in the book's index for "nationalism" (and searching inside it in Google most uses of the term are discussing Napoleon's early Corsican nationalism or burgeoning nationalist movements in countries France occupied in this time) so I'll pick a different topic: art in the Napoleonic era. Let's say I want to find some primary sources on that. So I look in the index under "art" and find a reference, on pages 543-5, to "arts, N's patronage of." Skimming that section turns up a number of quotes, and endnotes numbered 84 to 93. Those endnotes list a number of sources; most of them happen to be secondary, too, but a book like Empire Style and Napoleon and His Artists likely have a lot of direct sources. One of those books might quote someone in a letter or a memoire; you can then search to see if the full text of that letter or memoire is available online or in a library.

Another option is to find an oral history-style book that is a collection of eyewitness accounts. For example, Paul Britten Austin wrote a three-volume history of the 1812 invasion of Russia and a book on Napoleon's march from Elba to Paris at the beginning of the Hundred Days; both are brimming with quotes from letters and memoirs that you can then mine as primary sources. Austin's 1815 book is part of a series called "The Napoleonic Library," which is also largely eyewitness accounts. You might find useful books in it such as From Corunna to Waterloo: The Letters and Journals of Two Napoleonic Hussars, 1801-1816, or The Note-Books of Captain Coignet: Soldier of the Empire 1799–1816 or Memoirs of a French Napoleonic Officer: Jean-Baptiste Barres, Chasseur of the Imperial Guard.

In summary: start with the sources you have. See who they cite. If you keep going up the chain you'll eventually reach people citing primary sources. (If not, the sources you have aren't very reliable.) Look for published versions (especially in translation, if you're covering people who spoke a different language than you) of contemporary memoirs. The same author or publishing company that publishes one such memoir or letter collection is likely to have published others, too. Any source you do find, even if it's not quite what you're looking for, can lead you to another source.