Cognac is a double-distilled brandy from Cognac, France. Cognac is graded by aging, 3, 5, or 7 years. According to Sharon Tyler Herbst "Label terms X.O., Extra and Reserve usually indicate a Cognac is the oldest a producer distributes."
You can purchase Cognac in most liquor stores or grocery stores with a well stocked liquor section. Additionally you can purchase this online at Bevmo.com. Courvoisier is a well-known brand that ranges in price from $25.00 to over $500.00 a bottle.
If you don't have Cognac you select one of these substitutes:
For Cooking If you use alcohol