Widening them or modifying them from 14" to 15" isn't really any different than modifying regular steel wheels - the center is taken out and mounted inside a new rim, with the desired width, diameter and backspacing. There are a number of places around the country that can do the work. One place where they're available is BOP Engineering:
https://www.bopengineering.com/pontiac_full_size.shtml