You generally need to talk to a geo-services company like ESRI for something like that. I don't recall seeing databases with boundaries coordinates (there could be dozens and dozens of coords for a given zip code), but I _think_ ESRI can do it, and they probably have a web service for it.
esri.com, they have a lot of different things so you may need to talk to someone to narrow down what you want.
-- gw (www.railsdev.ws)