Primary sources can be such a wide range of document types, from newspaper articles to letters. There is no comprehensive website for finding them. Here are some suggested starting points.
Primary sources like letters, diaries, government reports, and other documents can sometimes be found published in books. Sometimes, documents included in published collections are translated, which can be helpful if you don't read the language spoken in the area you're hoping to study. To find sources from a particular country, search for the country in Tripod then limit Books & More results to the genre "Sources."
There are many websites offering digitized versions of primary source documents pertaining to Latin America. Here are just a few options. Talk with a libraran to find others.