For those who plan to use Centric/StopTech rotor replacements, here is a quick breakdown of their part numbers, with a link to their online catalog to find the correct part number for your Mustang application. Many in the TMO community use Centric rotors for heavy duty track usage. Here's one thread discussing this topic. Search the forum to find several others.
121.xx is the "C-TEK" econo-line uncoated plain rotor
120.xx is the Premium series which have black coated centers
125.xx is the "High Carbon" Plain rotor
320.xx is the newer "GCX" Elemental Protection coated series
Model numbers change depending on whether you have a Performance Pack, Ecoboost, etc, so be sure to pay attention to the Sub Model.
Link to Centric's catalog:
Search | Centric Catalog
That link takes you to a results page for a 2018 Mustang, but you can easily change the year in the left column, and choose your Sub Model as well.
You can then grab that product number and check if Amazon has them available for fast Prime delivery, or check other online retailers like TireRack, etc.