El Salvador Declassification Project: Declassified documents from the U.S. Department of State, as revealed by Freedom of Information Act requests.
Radio Venceremos Audio Collection: https://hrdi.lib.utexas.edu/hrdi/mupi/
CBUES Repository (El Salvador): This repository provides access to collections digitized by libraries and Universities in El Salvador.
Hathi Trust: https://www.hathitrust.org/: A collation of libraries have joined together to digitize public domain materials.