Taxonomy for state city zip

Hi,

I want to create a wordpress website for a client which would help people to find a zip code for a particular city, So people would select a state from the drop down then next dropdown would get populated with cities of the selected state and after selecting the city, the last dropdown would be to select the area of the selected city for which the user wants to see the zip code.

Client has all the data with him, like the list of more than 50k zip codes which are mapped with areas / cities / states, I know this could be done with custom taxonomy and custom posts but need guidance to get this done.

Please help.

Thanks,

Trapti