GADM (Global Administrative Areas) v3.6 provides spatial boundaries for countries and their administrative subdivisions (states, provinces, counties, districts, etc.). Data are available as shapefiles, GeoJSON, and R spatial objects and are commonly used for mapping, spatial analysis, and joining to attribute data.
database is version 4.1, many researchers and GIS professionals still require Version 3.6 download gadm data version 36 work
usa_shp <- st_read("gadm36_USA_1.shp")
You can access the data through several official and community-supported channels: GADM (Global Administrative Areas) v3