A number of government and intergovernmental websites may prove useful.
The World Trade Organization has a tool called Tariff Analysis Online (though you do need to register to utilize it).
The United Nations Conference on Trade and Development (UNCTAD) maintains a database called TRAINS, which they describe as "a trade and market access information system combining data on trade, customs tariffs, and non-tariff measures. TRAINS contains HS-based tariff data for over 170 countries and for several years."
And the United States International Trade Commission maintains a tariff database, among other things.
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