start here http://opendata.stackexchange.com/a/1104/1876
NCES - Identification of Rural Locales http://nces.ed.gov/ccd/rural_locales.asp#defs
Urban Areas - Cartographic Boundary File: http://www.census.gov/geo/maps-data/data/cbf/cbf_ua.html
Rural Definitions: Data Documentation and Methods http://www.ers.usda.gov/datafiles/Rural_Definitions/StateLevel_Maps/CA.pdf