There are many lists of links to Latin American newspapers, one excellent one is listed below. Its only drawback is that it does not tell you which are the main newspapers. The Nations online/World News site is more selective.
Today's Front Pages from the Newseum lets you sort by region. If you choose to look at the South America region, you can discover which newspapers they are featuring. these are likely to be significant publications. You can click on the link for a larger view of the front page and from there find a link to the newspaper's web site or you can search, using the newspaper's name and location.
Databases that include Latin American newspapers are listed below.