>If you have a data file "ziplnglat" that lists all the zips with their long and lat (purchased)

Not necessarily purchased - what you pay for is also available for free from the government websites. I've googled it out once a couple of years ago - and if it was found once, it can be found again :). I think the words to search for are some of "zip database survey" or something of the kind. Also, the site was something *.gov.