This is a dup of my reply on DU3
if your recipe actually reads "egg nog flavoring", then I think you're looking for an ingredient called "egg nog flavoring" rather than something that adds flavor to egg nog.
Wait, that's a confusing way to say it.
There are bottles of flavorings, similar to bottles of vanilla flavoring, orange, lemon, rum, etc. There are actually dozens of them in all sort of varieties, like butterscotch, fruit flavors, and some complex flavorings like bordeaux, red velvet cake, princess cake... and egg nog, among others). They're highly concentrated flavorings and tiny amounts can bring forth a lot of flavor. If your recipe calls for a small quantity of (in the range of teaspoons, or less), I have a hunch this is the type of product it's written for, as opposed to a more liquid product like rum. A concentrated flavor like this will bring the whole richness of eggnog along with the eggnog spiciness to your cookies
Okay, if that's what the recipe wants, where do you get it?
I can't specifically tell you, since I don't know your area. But I do have some suggestions as to where you might look. Try gourmet markets, cooking stores like Williams-Sonoma or Sur la Table, some supermarkets carry stuff like this, candy-making supply stores. Amazon and king arthur flour have it.
http://www.kingarthurflour.com/shop/items/eggnog-flavor-1-ozhttp://www.thebakerskitchen.net/eggnogflavor.aspxhttp://www.amazon.com/faeries-finest-eggnog-flavor-extract/dp/B002X3RND4Have fun with your holiday baking!