The process isn't difficult -- finding the time to do so is.
I downloaded a database of all US zipcodes, and their Latitude/Longitudes. Then, calculated the distance between every combination (using formulas in the
zipdy code)
The data isn't ready to go yet, and with work as hectic as it is (12+ hour days, 6+ days a week, till late December) I won't be able to get it working. I'm guessing around Christmas you will be able to search by zipcode.
I may end up using that data for other stuff too, who knows. It sure did take a long time to calculate and upload (7GB CSV file for distances < 500miles)