How to get all USA states and their cities

Hi, I am working on a web program using ASP.NET.

 I would like to add a dropdownlist box where all 50 states in USA will be shown, and if I click one state, its cities will be shown in the other dropdownlist box. My question is : how to get all USA states and their cities using a program, like a webservices? Does any government website provides a XML file in which we can get the states and cities? Thanks.
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