Topic awaiting preservation: zip code lookup (Page 1 of 1)

I have a client that has a list of vendors and their full addresses in a table. They want to have a form created that submits a zipcode and returns all vendors located within say a three hundred mile radius. Now I know simple CRUD, yadayadayada, but can anyone suggest a method of relating vendors in a set specified radius without having to go figure it out and add it to the db record?

How do I search based on zipcodes
Searching with location radius