Sometimes it helps to go back to the beginning. Here's the original paper published by Cook & Torrance
Their paper outlines using full Fresnel equations but doing so is computationally expensive. Christophe Schlick developed an approximation method for the Fresnel Factor that's much easier and faster to generate results with less than 1% error so this Schlick Approximation is what's used instead almost exclusively today. The next link is to his paper.
Triem23 found a great IOR list somebody on another forum put together
I've taken part one of that list and gotten it into an Excel sheet so you can always have a copy handy