On: USPS: ZIP Codes are “proprietary business information”

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Datasets:

  - United States Census Bureau TIGER
  - GeoNames data dumps
  - You can pay to be sent data files from the USPS (AIS)

Welcome to the wonderful world of public, semi-public, and private datasets. It is an interesting niche, and the ability to find them can give you a competitive advantage (e.g. machine learning datasets).