Find the user's zip code

I am creating a new app that will allow users to search for local concerts. This app will pull data from a web database that uses a supplied zip code or state to determine the market area that should be searched.

What is the best way to determine a user's current zip code? I can't seem to find much documentation on getting information from the user's network location - how to do it, what format it is in, ways to convert that to a zip code - all the samples seem to use the GPS, which is overkill for this app. Any suggestions?