Two of the best bet starting places for economic data through the Libraries (including GDP figures) are the International Monetary Fund Statistical Databases and the OECD iLibrary. Both contain a wide range of economic indicators and some time series will go back as far as the 1940s or 1950s, although availability of indicators and date range varies for each country. United States' economic data is available via the Bureau of Economic Analysis, as well as from the U.S. Census Bureau.
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