It's going to depend on why you're looking for a substitute. If you're an alcoholic (like me) or are looking to make this as a minor, you can potentially try an alcohol-free brandy. They're not a straight substitute and you wont be able to flambé. If you're religiously required to abstain from alcohol or alcohol products, you're likely not going to find a substitute. Most NA variants of beer, wine, or liquor started out as alcoholic. This means that if, for instance, you were muslim, most of these would still be haram. If you're just looking to use something with some of the same sorts of flavors, I'd suggest "Eau De Vie". Its a non-alcoholic brandy. You WONT find anything that says "NA Cognac", as cognac is a protected term (like champagne). Brandy is to cognac as sparkling wine is to champagne. As a recovering alcoholic who made his entire professional life in the production and science side of alcohol, I genuinely hope this helps.
