Ninety nine percent of the time you'll get a "good" rotor from a parts store. I'd pick one with a medium price range. Or, if you're super concerned, buy one from your local Ford dealer. Being a retired professional mechanic for over 50 years I can count the number of bad rotors I've bought/installed (via my employer) on half of one hand. And that covers the gamut from the cheapest to the most expensive.