I try to work with geospatial data of North America. Is there any high-resolution map material available for/in R?
The R package rgdal has functions for reading in data that can be read by the GDAL (for raster) and OGR (for vector) libraries. This includes Shapefiles, GeoTiff, GML, Arc/Info Grid, GPX, GeoJSON etc etc etc etc. I could go on.
If you can't find the data you want in one of the formats that R can read via the rgdal package, then I would be very surprised. You don't need special map data for R, you can use almost anything.
Read the R Spatial Task View for everything you need to know about spatial data in R.
Have a look at maps ( http://cran.r-project.org/web/packages/maps/index.html ) and mapdata ( http://cran.r-project.org/web/packages/mapdata/index.html ) for higher resolutions.