The raw data before 1987 is downloaded from the National Archives. Thereafter the data come from the Census CBP website. Links:

National Archives Website: https://research.archives.gov/id/613576
CBP Census Website (post 1985): https://www.census.gov/programs-surveys/cbp.html

The zip file for each year contains the national, state, and county file and, if available a industry and geography reference file for the given year.