on the world market, any finance site will do. the most straightforward for me is Yahoo! Finance.

are you looking to invest in commodities, or just looking for how much you should spend to buy rice or corn for your personal consumption? because the finance sites will help you with the first, but not the second. for the second, mamalengke ka na lang

if you're looking to invest, be warned that commodities are very volatile. aral muna bago magbagsak ng malaking pera. think about market conditions and the likely impacts on supply and demand for the product. we all know about oil, but gold is a well-known hedge against currency volatility/economic downturns, copper and steel are most likely in demand with all the telecom and real estate development that is happening in the RP these days, and so on.