Many universities have Libguides relating to Latin American History - just search "Latin America (or your country) primary sources libguide". Some of the links will be to open access sites; however some will not be accessible as they require you to have a university log in.
There are three books with primary sources for Mexico. They are in the Primary Sources collection opposite the library front desk.
If you cannot find a primary source after looking at all three books, come and talk to Hayley or Nicole.