Zip Code reverse lookup

I have created a feedback form for visitors to an airshow and the results are shown here http://hollisterairshow.com/feedback-results.php. I would like to add a column beside the zip code showing the city. On looking around I have found several free zip code lookup sites and temporarily added a link to one of these on the feedback results page,  but I think my users would find it more useful if I just show the city alongside the zip code. I'd really appreciate suggestions on how to do this.

Thanks.

Tony