Emily likes Diamond Crystal brand. Other brands of salt may be finer in texture and not as much by volume will be needed.
Clarified butter lacks milk solids, so it won't burn at high temperatures. You can buy it (sold as ghee) at large supermarkets or make your own easily, following online directions. Or just use oil: It's less flavorful, but you'll be buttering the popovers at the table anyway!
A Word on Pans
You can buy pans specially designed for popovers. They look like six-cup muffin pans, and the cavities are deep and narrow. If you have one and would like to try this recipe in it, divide the batter evenly among the cups (it will reach nearly to the brim) and plan for big popovers.