There are three types of duck breast that you’re likely to find at your market. The white pekin (a.k.a Long Island) duck boasts tender, mild-flavored meat. Muscovy duck breasts tend to be slightly larger and more strongly flavored than white pekin. Moulard ducks, which are a cross between a female pekin and a male muscovy, are mainly raised for their livers (foie gras), but moulard breasts, called magrets, are considered a delicacy by those who prize their large size and steak-like eating qualities—and are willing to overlook their occasional tendency to be stringy.
Most ducks are sent to market when they're still young, so expect the terms"duck" and "duckling" to be used interchangeably. White pekin is the most common variety in the average supermarket, followed by muscovy. If the package doesn't specify a variety, chances are that it's white pekin. Moulard breasts are more of a gourmet-store specialty item. The recipe Pan-Roasted Duck Breasts with Sherry, Honey & Thyme Sauce was developed using pekin duck, but it will work for muscovy and moulard, too; just be prepared to increase the coooking time a bit.
For mail-order muscovy and moulard (magret) duck breasts, contact Joie de Vivre (209-869-0788). For white pekin duck, visit Mapleleaffarms.com.