Official source for U.S. export and import statistics
What are Mirror Statistics
Ideally the import reporting of country A from country B would match the export reporting of country B to country A. This is often not the case.
Sometimes there is no data from a country. Aggregating sites (such as the UN) may recreate data using information from similar or trading partner nations. The reporting site should note that the data is estimated and not original.
Tips for Hard to Find Data
If you cannot find the import data for a country, look for the export data of their trading partners (or vice-versa)
Look for the country's official statistical website (use Google translate if necessary)
Look for international professional or industry organizations for the specific trade category, for example agriculture, automotive, textile, etc.