Calculating distance between Zip codes

All:
I am trying to add a feature which would allow users to look up some certain stores within a certain distaince. for example: users should be able to enter a zip code and 10 miles as the distance then click on search and the page should show a list of walmart stores within the 10 mile radius. To this end I have the following questions would like to seek your advice on:
1.) How can one obtain a list of zip code in US?
2.) How do I calculate the distance between zip codes?
3.) Is there an existing web-service I can leverage?
thanks a million,
James Chen
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