Dadra,
A bit off the WordPress topic, but you may want to consider using the US Census API to do your lookups.
Integrating this into WP should be pretty simple, could be done by zipcode, city name, etc… and could be done with and AJAX callback all on one page.
Example of what I think you want: http://quickfacts.census.gov/cgi-bin/qfd/lookup
US Census API Developer site: http://www.census.gov/developers/
For specific WP implementation methods, I’m thinking you may want to hire someone with a fairly solid background with external API integrations.