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Personally, I would use Google Maps and a proximity search.
Here’s a tutorial: https://developers.google.com/maps/articles/phpsqlsearch_v3
In order to realise this you will need to geocode your zip codes into longitude/latitude coordinates, interact with the GoogleMaps API, parse and display the results, not to mention have a working knowledge of PHP and MySQL.
To me, this seems somewhat more complicated than you realise.
Here’s another link that might be useful: http://stackoverflow.com/questions/2510514/find-nearest-location-by-zip-postal-code
Great, thank you for the help. I know its not simple, which is why I said “seems” simple, doesn’t mean it is.