An increasing number of primary sources are available for free on the web. Here are some search strategies for finding them:
Strategy 1: To focus on primary sources, go to a search engine like Google, and combine your topic with terms designating primary sources such as memoirs, diaries, accounts, narratives, documents, autobiographies, online archives, correspondence, speeches. You can link synonyms together with a capitalized OR. Here are some examples:
"Pancho Villa" "primary sources"
"Mexican Revolution" accounts
Mexico "online archives"
Strategy 2: Another great way to find sources on the web is to think of a library, archives, or university that specializes in your topic, and pay them a "virtual visit" (check their web site for online collections).