When I had a 7.3 idi Ford, I just went to my local auto parts store and asked for glow plugs for it. Had several different brands over the years, all lasted and worked about equal.
No matter if you have an IDI or a PSD there is only ONE brand of GP to use, Motorcraft/ Beru brand. They will be stamped or etched "Made in Germany" or "Made in Ireland" and have the name "Beru" etched on the side. Not Bosch, Autolite, or any other brand. Auto Zone carries them as well as Ford. They should be about $9.50 ea. All the other brands have a tendency to have issues later on.
Quick question...do these glow plugs have a tendency to swell up like the GM 6.2 Diesel's GP's when they get old?