How to retrieve conversion factors from internet?

FROM : KSamdev
DATE : Sun May 25 21:21:45 2008

Hi,

I am writing Converter Application in Cocoa and would like to retrieve 
units conversion factors from internet (at least such as exchange 
rates that change all the time).

Anyone is aware of any such server available? Is it possible to get 
such data from Google, Yahoo? How?

Thank, Sam.