If you only have a few communities that you serve, entering data manually may be the simplest approach.

Geonames.org provides free ZIP code to city, state and county information licensed under the Creative Commons Attribution 3.0 License, which means you need to put a link to them on your website. Their data includes primary city, state and county information only. It doesn’t include info about which other cities are included in a ZIP code. Visit http://www.geonames.org for more info.

The following code example shows you how to download and reformat the data into the zips.txt format. You have the option to filter the data to only include certain states also.

## How to get a generic Evergreen zips.txt for free
wget http://download.geonames.org/export/zip/US.zip
unzip US.zip
cut -f2,3,5,6 US.txt \
| perl -ne 'chomp; @f=split(/\t/); print "|" . join("|", (@f[2,1,0], "1", "", $f[3], "")), "|\n";' \
> zips.txt

##Optionally filter the data to only include certain states
egrep "^\|(ND|MN|WI|SD)\|" zips.txt  > zips-mn.txt

There are many vendors that sell databases that include ZIP code to city, state and county information. A web search will easily find them. Many of the commercial vendors will include more information on which ZIP codes cover multiple cities, counties and states, which you could use to populate the alert field.

Another possibility is to use your current patron database to build your zips.txt. Pull out the current ZIP, city, state, county unique rows and use them to form your zips.txt.

Small Sites. For sites that serve a small geographic area (less than 30 ZIP codes), an sql query like the following will create a zips.txt for you. It outputs the number of matches as the first field and sorts by ZIP code and number of matches. You would need to go through the resulting file and deal with duplicates manually.

psql egdb26 -A -t -F $'|' \
 -c "SELECT count(substring(post_code from 1 for 5)) as zipcount, state, \
 city, substring(post_code from 1 for 5) as pc, \
 '1', '', county, '', '' FROM actor.usr_address \
 group by pc, city, state, county \
 order by pc, zipcount DESC" > zips.txt

Larger Sites. For larger sites Ben Ostrowsky at ESI created a pair of scripts that handles deduping the results and adding in county information. Instructions for use are included in the files.