The two main sources for this information in the United States are the Federal Communications Commission and the U.S. Census Bureau. The FCC has information on " fixed Internet access services connections at the county level" via Form 477. Similarly they have information on broadband access via the same form, but only down to the state level. (All facilities-based broadband providers are required to file Form 477 with the FCC twice a year regarding where they offer Internet access service at speeds exceeding 200 kbps in at least one direction.)
The data from the U.S. Census Bureau comes mainly from the American Community Survey (ACS) and the Current Population Survey (CPS). If you wanted to explore this data in aggregate before digging into it too deeply, you can access data from the ACS via the Libraries subscription to a database called Social Explorer. There's an open data source called IPUMS that will provide you with microdata for the ACS and the CPS if or when you're looking to see more granular data.
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