Calvados is a type of Brandy made from apples. Calvados is a specialty of Normandy, France. Aside from being a sipping branding it is also used in cooking.
You can purchase Calvados at any well stocked liquor store or purchase it online at Bevmo.com
If you don't have Calvados, you can substitute equal amounts of:
Armagnac OR> Applejack OR> Pear brandy OR> Equal parts of apple juice concentrate and cognac OR> Per 1/4 cup needed use 2-3 teaspoons brandy extract + 1/4 cup water or other liquid in the recipe. OR> Unsweetened apple juice concentrate, apple juice or apple cider OR> Apple butter