Here is a script that I use all the time but you need to find a zip code database with latitudes and longitudes. Google it and you'll find it. This script will work with SQL database, access db may be bit different due to math.
<cfquery name="passedzip" datasource="db" maxrows="1">
SELECT zipcode, latitude, longitude FROM ZIPCODERADIUS
WHERE ZIPcode >= '#zip#'
</cfquery>
<cfquery datasource="db" name="getZip" >
SELECT zipcode
FROM ZIPCODERADIUS
WHERE 3963 * (ACOS((SIN(round(#passedzip.latitude#,3)/57.2958) * SIN(latitude/57.2958)) +
(COS(round(#passedzip.latitude#,3)/57.2958) * COS(latitude/57.2958) *
COS(longitude/57.2958 - round(#passedzip.longitude#,3)/57.2958)))) <= #URL.radius#
ORDER BY zipcode
</cfquery>
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