How To Get A List of All Zip Codes In Two Counties?

Does anyone know of a place to download a list of all zip codes in Boise and Elmore Counties, Idaho?

This page gets you zip codes by city--but not for an entire county.

UPDATE: One of my readers used his skills with GIS to get me a spreadsheet with all the zip codes for those two counties.

UPDATE 2: Here's a page that will let you get a list of all zip codes in any county in any state. Unfortunately, it isn't very accurate!