Prices for commodities such as agricultural products, chemicals, petroleum products, and metals are often available via exchange data:
Bloomberg has a wide variety of data available for the commodity markets, including current market monitors historical data.
Workspace has good coverage of historical spot commodity prices worldwide, as well as some futures and options data (including expired options for 1-3 years).
The CRB Commodity Yearbook is produced by the Commodity Research Bureau and provides an overview (including prices) of a wide range of commodities.