Where can I find a shapefile of Central America + Mexico (WSG84) (with no administrative boundaries) at a low resolution?
I'm looking for a very simple shapefile without the small islands and smoothed out coastlines.
Frequently updated OpenStreetMap data licensed under OdbL might be an option for you. They can be downloaded from http://openstreetmapdata.com/data/coastlines in WGS84
Here you can find coastline data at different resolutions and quality stages: A Global Self-consistent, Hierarchical, High-resolution Geography Database.
You will find Mexican shapefile free to download at Effortless Online GIS Mapping - Mexico shapefile. You can also convert this shapefile in KML or CSV