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You can buy a commercial zip-code database for $300 or less. One such vendor is Zipinfo.com. Check out their ZIPList5 Geocode database. That looks like just what you need. Once you have it it will generally include latitude and longitude of the center of each zip. Once you have a ZIP to latitude/longitude database you would either:

find all zip-codes whose centers are within X miles of the user-entered zip, then find all stores in those zips.
find all stores within X miles of the user entered zip. This will require pre-determining the latitude/longitude of all your locations (you can pre-compute the aprox latitude/longitude of each store from the zip-code DB).
One approach may be faster than another based on how many stores you have. Unless you have as many locations as McDonalds the second option is probably faster.
Either way you're going to have to compute the distance between 2 points of latitude/longitude which is not an easy formula to derive because of the curvature of the earth. However it has already been derived by many other people. Follow this link for details of how to perform that computation.
The basic algorithm above can be very slow because it involves computing many distances. However, it can be optimized significantly for actual implementation. It does however start you off on the right track.

In reply to RE: Searching by zip code by lhoward
in thread Searching by zip code by funkwater

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